Honestly its best if you just look up beyond divinity.
its a strange game, you have two characters, one of which is a paladin who you can fully customize and one who is a demon.
strange game but fun all the same.

On your points

1 - but they do. Pathfinder included "new" things but it also had the exact same system for them.
People dont like change. Even if its change for the better, same as they now hate on PF2.0
2 - I already told you the solution, give each class a specific niche, dont let one class overshadow another, have the classes NEED each other and work as a TEAM, i dont care about the MMO boogeyman. MMOs came from MUDs which came from online PnP sessions, just because MMOs do soemthing doenst mean its automatically bad, or generic for that matter.
Roleplay is one thing, but people will find justifications to powegame.
You have it the wrong way around. The rules should be balanced, the DM should allow unbalance if its in the interrest of roleplay. What you suggest is that the rules should be unbalanced and people should self regulate.
i dont think this will ever happen.
This is ok in a single player PC game, but not as soon as several people are invovled. Its just not fun to be the party face trap disam rogue and just have the wizard do all those things with a spell that you built your character around.

3 - well, its a matter of taste. In a genre mostly known for dungoen exploraiton id say a long range character is pretty niche.
I think the dragons dogma case is a chicken or the egg question.
Is the Ranger a popular class in Dragons Dogma because people want to play a "sniper" or is the Ranger a popular class because in Dragons Dogma its fun to use.
In Everquest Necromancer was one of the most played classes and it bareley exists in PnP RPGs and CRPGs, mostly as some obscure prestige class in NWN2.

A game has to decide what options ot include and what options not to include.
It doesn tmean that a game SHOULDNT include a sniper class, all im saying is that OS2 decided not to. but OS2 did other things that are "niche". Summoners are not very common in RPGs, and neither are wand and shield casters, which is a great option in OS2.

On casters:
Spell slots that the casters can allocate EVERY 8 HOURS OF REST.
meanwhile a non caster needs to build his ENTIRE CHARACTER to do this ONE thing and CANNOT change it even on level up.