I fundamentally disagree with your stance on multiclassing.
Archetypes? I'll make my own damn archetypes, thank you very much! I'm not interested in playing a cardboard cut-out clone of everyone else's paladin (Where's your plate armour? You're not a
real paladin without plate armour!), warlock (No, all warlocks are evil in
my world!), barbarian (the only allowed background for barbarians is Outlander, otherwise you're not a
real barbarian!), etc.
Unnecessary? Sure. Just like rogues are not necessary. Just like sorcerers, bards, greatswords,
mage armour, skills!
None of these are 'necessary'! If we banned everything that was unnecessary then we'd just have fighting men, wizards and clerics, all weapons would do 1d6 damage, and there is no need to level up at all!
Implying that, automatically, multiclass PCs do not have clear and distinct abilities. Strange, when I look at the character sheets of both single and multiclass PCs, each has a list of their class abilities. No difference at all in the clarity of these abilities.
Distinct? In what way? Making every fighter identical to every other fighter? Multiclassing makes for far more 'distinct' individuals than single class PCs in terms of abilities possessed.
The opposite is true. Characterisation comes from
me not the book! If we all had to play the same 11 classes, our PC would not stand out at all!
What you are talking about is that each
class would be played differently. Sure. But that is a circular argument: "It's easier to play each class differently because each class is played differently". But I'm not remotely interested in playing a 'class'; I'm playing a 'character!
I'm totally happy with my PC. It plays the way I want it to because I chose (within the limits of the RAW and those set by the DM) each class level and the abilities that went with them. I did this knowing exactly what I was doing.
It makes it easy for brand new players to play single class PCs because the thought that goes into creating a PC
that works has been done for them. You really have to know what you're doing to make an
effective multiclass PC. But I've been playing for 40 years now; I've got the hang of it!
I have another take on this: D&D is a class based system. That has strengths and weaknesses. Multiclassing, done well, can eliminate (or greatly mitigate) those weaknesses, leaving the strengths of the class based system intact while gaining the strengths of classless systems too.