Fewer classes are better for a few reasons.
This will be fun to dispute. Okay....
First, it prevents unfamiliar ideas from taking over the table. No matter how many splatbooks are released the classes will have some familiar elements, the warblade will still do something fightery, the shugenja will do wizardly things, and the like. This makes the subclasses feel familiar while being easier for the DM to remember what the class does, and easier to learn.
A. Part of the problem is how you define "wizardly things". Also, why should the "shugenja" do "wizardly things" in the first place?
B. How does it make things "feel similar"? Consider 3e's run, you had all the classes in the splatbooks, say Complete Arcane, and they all followed the 3e math for casters. But those splatbook casters did not receive the same amount of attention nor were they very good. Having "similar feel" in math for the system didn't help me get into psionics even if the spells/powers were very similar in power.
As a related point, it prevents classic classes being made redundant from a story perspective. So many 3e classes were variants on the core classes with a slight, slight story tweak. And many 4e classes were established classes with a role tweak. To make room for the invoker, you had to narrow what it means to be a cleric. When you describe a "heavily armoured warrior skilled with a sword" or "arcane spellcaster in robes" only one class should print to mind.
C. You don't need a class/subclass system to achieve your second paragraph. Name all casters "casters". Name all who use weapons or skill [monkeys] such. How does a class/subclass system help here?
It also makes new options easier to describe. Rather than having to start from scratch describing what a "dragonfire adept" or "archivist" is, you just need to mention it's a subclass of ______ and half the work is done.
D. HOW?
Wouldn't it be easier to describe them in terms of their ability? Dragonfire adepts (from experience) are going to be relatively terrible wizards. They are not scholarly. They don't brew potions very well. They are much closer to warlocks and while warlocks have some wizardy (technically sorcerous) ability they are not the same basic archetype. Archivist fits much better as a wizard type, but from play experience I would guess their supertype would be priest? Why are you defining the characteristics (NOTE: "Character") of the class by the mechanics of the class in a roleplaying game? If the dragonfire adept happened to channel white magic/divine magic as opposed to dark/arcane, how would you define it then? That could be an easy change as far as "subclasses" goes. A VERY easy one that might have huge impact, or very little. A psion is a very different creature in terms of power structure, power manifestation, background/description and general mood of the character; when defined along with a wizard, or sorcerer, or even dragonfire adept.
As (I think KM) said: What makes subclasses BETTER at doing this as opposed to just different at doing it? If you are going to end up with the same giant lists of subclasses/classes why is it better in this format?
Second, it's also good for balance. It prevents future options from becoming too overpowered, as they're not making an entire new class from scratch.
And it prevents new options from making old ones redundant. Pathfinder has that with the ninja being a better rogue than the rougue, and 4e had this with all the second gen controllers being better at controlling than the PHB1 wizards. Instead, if a class is underperforming, they can release better subclasses keeping a core option relevant.
Except I play pathfinder. Do you know what happens when you play a ninja/rogue?
E. You lose something, you lose basic powers that (if you were a straight rogue) you would have otherwise kept. That happens with all the pathfinder substitutions. If an option were to be the other way around, the option being better than the default, then no one would take the default. So either make the subclass weaker or stronger, and invalidate whichever option is weaker. There may be some who will pick the weaker version for RP, but they will immediately recognize that. They would have also done it if it was its own class.
F. It hurts you to multiclass. It isn't a big thing by itself, but often times if you want to multiclass into several different classes (not talking between rogue and ninja) you lose something or are set further back than if they were different classes.
G. You can't level dip to pick up normal rogue abilities. It is simple. If the ninja loses.. let's say evasion or trapfinding that the rogue gets at low levels then you might want to dip and pick those abilities up. You can't when they're subclasses because you can't dip into the class you are already in.
H. It is another choice point that you have to make. Meaning that if you are playing a ninja you are going to be as similar to ninjas as every other ninja. If you could normally be a rogue (bland, regular), rogue-thug, rogue-ninja; and you choose rogue-ninja, so does every other rogue-ninja. You are then the same as ever other rogue-ninja because everyone had to make that choice point. You lose something there, you can't be a ninja-thug, or whatever the ninja variants are. That is a choicepoint you lose with a subclass you don't lose with a CLASS.
Third, it allows for better support. Everyone remembers the classes that saw no options in 3e beyond their one source book. 4e tried to counter this by declaring "everything is core" and thus worthy of support in future accessories. But there were quickly too many classes for equal coverage.
Actually it is the same problem. Good splatbooks in this way will always try to support characters or provide backward compabability. Bad systems/splatbooks won't. The problem with the binder wasn't that it wasn't a wizard subclass, it was that it sucked and that it never got any further support. Making it part of the mage superclass will only drive the people who play a binder crazy, as it never gets further support down the line either way.
Subclasses mean all future books have a finite number of classes to generate content for. Every feat, magic item, or spell aimed at a wizards applies to almost all versions of the wizard.
While this is nice and what should hopefully happen it isn't realistic. Just like already shown, first DMG has pearl of power/boccobs blessed book. Those items are exclusionary to sorcerers. I know you said almost but that is what we're talking about here. Wizard/Sorcerer were about as close as we can hope to get in terms of power aligning with spells and even then items can only be made for one or the other. I guess I just want more idea what you are expecting to happen or how they stop this kind of thing from happening outside of saying "subclasses" will fix the problem.
Fourth, it takes less space. 5e classes can take less room than a 4e class, but they're still not light on the page count. You get more diverse content if you diversify via subclasses. You can have three or four subclasses in the space it takes to have a single class. Even if you keep the same fluff:crunch ratio as the late 4e books, you'll effectively get more options though subclasses.
I've seen entire systems shorter than the classes for 4e. Being short has little to do with the subclasses. If anything putting subclasses under the base class section will increase the length. I think you are over attributing one aspect of 5e with one part of it. 5e is shorter.. because of subclasses? Are subclasses also responsible for the way spells are worded?
Why? You think something is depending on you getting your position out every time? What's driving you to make the same point over and over again? You know, there's no vote being counted here with each new thread.
I don't know why I'm injecting myself into this conversation. But here goes.
I went and looked at those threads. His arguments were similar. But they were hardly the same. They addressed the topic of the conversation. I guess part of me wants to know, in that case, why YOU look restating your position every time. You are frequently the first response after FI and say very nearly the same thing ever time. Yes we know Mage is an organizational thing. (Except it may not be, we don't really know what it will look like yet.) So.. "
What's driving you to make the same point over and over again? You know, there's no vote being counted here with each new thread."
You're doing it repeatedly, for compliments? Others have said they've slapped you on their ignore list over it. Were the compliments worth losing all communications with multiple people over you doing this?
Have you looked at the XP he (yes, I'm assuming FI is a he, even if he isn't) has? They're pretty universally "hey good job/+1 for insight" kind of stuff. No one saying "ignore now". Not one.
Maybe. Maybe not.
But, I could make the exact same statement regarding the "wizard" which is being given equal footing as a subclass. They might easily focus on sorcerer or warlock schools and leave the wizard under supported.
Agreed. I think the problem here is that ANY subclass could be under-supported. It might be under supported because the overclass might focus on one branch or another. If Sorcerers become the most exciting class at release then it is quite possible wizard, or psion or any other subclass might get ignored to the detriment of that subclass. There are certainly more suppliments for Barbarian or Paladin in 3e splatbooks than for Fighter or Ranger that I ever saw. That is the problem when all four classes become a single class. Then add in that the game designers may look at the stats for "warrior" created stuff and say "yep, there's already a lot out there, job's done" so that fighter, or whatever, gets ignored
because the superclass isn't. I think that was @
KaiiLurker 's point.