Lovely, now comes the 3rd party flood of substandard prcs for every damned thing. I was much happier with archetypes. I feel they filled that niche much nicer than prcs.
If by archetypes you mean subclasses, then they serve a different purpose because they are fundamentally locked to a single class.
IMHO subclasses should be used to represent something
non-elite, just one possible path of the many for that class; by contrast, prestige classes should be used to represent
elite paths which are restricted by membership. For example, I am not sure that Purple Dragon Knights were a good idea as a subclass (but then if only Fighter are supposed to be PDK, then it's good!).
As for the danger of flood, it's there also for subclasses, if it hasn't started yet. WotC is
not going to bloat the 5e system with the current release plan, which is what so far, one sourcebooks
per year, plus small adventures additions? If it bloats with this plan, it bloats because of 3rd party products, so why is everybody asking for an OGL as soon as possible?
These, just like multi-classing, are not allowed at any games I run. Why? Because that's where the brokenness combos live, and character concepts get thrown out the window.
I agree that the latter should not be forgotten. For me even worse than breaking the rules it was to see ridiculous non-concepts like Fighter 4/Wizard 3/Rogue 2/Master of Shadow 1/Defender of the The Light 1/Fire Adept 1/Icemonger 1/Superhero 1/Extraordinary Commoner 1/Ultra Specialist 1/Mega Generalist 1...
Multiclass characters don't "instantly know something" any more than single-class characters who gain a new ability or characters who take a Feat. Don't treat leveling up as magic instant knowledge, but the moment at which your practice starts making a practical difference (quantified as a single moment for ease of use by us players).
Besides which, Fighter, Ranger, and Rogue are all skills any adventurer could develop; they are all practicing their skills and fighting. Sorcerer is natural ability awakening. Cleric, Druid, and Paladin can all represent a growing connection with nature or a higher force. The default flavor for Warlock seems to suggest that gaining the initial abilities is rather sudden.
I agree on both these points.
Although I have to say that I like the Rokugan fantasy setting because of its difference with the normal fantasy setting, that in Rokugan essentially
all knowledge flows through masters to students, with a strong emphasis on belonging to a school. You can't even mix base classes freely because if you're meant to be a Samurai, you join a Samurai school (when you're still a child) and you stay there forever. There is no such thing as "I want to learn a bit of magic, a few roguish skills, this special trick, that awesome technique" and cherry-pick classes, feats and other stuff. You have to stick with one path (Samurai, Shugenja, Monk or Courtier) and with what your Clan offers in that path, although elite options exists, but you have to earn access to them (and you are still normally restricted to your clan's), and once you enter an elite path, only a fool would leave it for another. Despite the abundance of published PrCls also in Rokugan, it was never a problem... unless you totally changed the setting's basic premise!