I fundamentally disagree with you.
Why have classes instead of a freeform system? There are a few good reasons:
- Classes allow for easy character creation with no system mastery necessary. All choices that need to be made are described in the class and, if it's designed correctly, all combinations of such choices result in a good characters.
- There are some archetypes important for the game - basing classes on them ensures that PCs will be relevant to the type of stories the game is intended for.
- Each class can be designed as a whole and, because of that, is much easier to properly balance than a freeform system where all combinations of abilities must be taken into account.
And here's the other side:-
* Classes may be easier, but 'easier' =/= 'better' for everyone. There are those of us who prefer complex characters, enjoy imagining and creating them, and like to think that the time spent on our hobby (and the resulting system mastery) can be rewarding. Whether our choices result in 'good' characters or not is partly dependent on our skill, and we wouldn't want it any other way
* Classes =/= archetypes; you can have, say, 'tribal warrior', 'mage' or 'scoundrel' manifest in infinite game mechanical ways without losing the archetypes. Plus, I'm not sure how 'important to the game' they really are
* 5E multiclassing is not a freeform system. You cannot get Ftr 5 before you get Ftr 4, for example, and you cannot get Extra Attack without having at least 5 levels in a single class, even if you have 20 levels of pure warrior-type class levels. Barbarian, Fighter, Monk, Paladin and Ranger all get Extra Attack at lvl 5, but a 20th level PC with pure warrior classes could be Bar 4/Ftr 4/Mnk 4/Pal 4/Rgr 4, and still wouldn't get Extra Attack. 5E is already balanced because
of the execution of its multiclassing rules
But then, by adding multiclassing, we lose most of these advantages.
Not everyone sees them as advantages, or agrees that multiclassing takes them away.
TL;DR: It's not about classless system being better than one with classes or vice versa. Multiclassing combines disadvantages of both with no significant gain.
Significant gains include:-
* Having more viable character choices without nearly as many artificial impediments. "What's that? Your concept is for a fully trained fighter who sells his soul to a devil? Impossible! You can either
be a fully trained fighter or
sell your soul, but there is no such concept
that includes both! I'm glad the rules don't allow it. Well, apart from the rules that do, but I'm disallowing the rules that allow it!"
* Just like any single class has levels 1-20 laid out, and as a PC gains XPs he automatically gains the next level and its associated abilities, a multiclass PC can do exactly the same. If your concept is for an Elven superspy a la
James Bond, based on a secret Elven Intelligence group called Lachrymae Shevarash
(the Tears of the elven god of vengeance), whose career path starts as a spy, becomes a field agent and graduates to full-blown assassin (gotta get that double-oh rating!), my pre-laid out 20 levels are: criminal/spy background, 1st rogue, 2nd-7th shadow monk, 8th-11th rogue/assassin, after that there are so few that each is unique in terms of what class they take at each level. Do I need to find a monk in-game before I need to gain the level 1 monk abilities? No. No more than the rogue has to find a rogue to gain the 2nd level rogue abilities. "But how do you explain getting monk abilities without instruction?" The same as I'd explain getting the next level of abilities in my original class: how does a rogue learn to assassinate without any meetings with assassins? "Ah, but we assume that a 1st level PC has been given enough training before the game starts that, with experience, he can master those pre-trained abilities such that he can use them in the field." Yep, mine too. All 20 levels were pre-trained and just needed the XPs to manifest, just like every other PC in the game!
* Some people don't like multiclassing. They don't have to. No-one is forcing them to multiclass if they don't want to. But it's not okay to have the attitude of "I
don't like it, therefore nobody else can have it!" I don't like playing clerics but I don't forbid anyone else playing one