A 20th level character is not suddenly more powerful in 5e than in 3.5. Somewhere deep inside we all have to know that. Just saying the editions aren't the same doesn't change that.
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thats kind of the point, in 3.5 for a game this long with this scope, to go to lvl20 would be weird. The same is true in 5e.
How long is the story of BG3, a few weeks? months at most. Possibly it's only days! Drizzt, the greatest hero of the realms, is level 19 after acouple centuries! of adventures bigger than BG3, you don't win in BG3 because you are the most powerful heroes in existence who have ever lived, the world trembles at your mention and the gods fear you, you win because you gather allies and have O's magic protecting you.
Level 12, fits the pacing of the game, it fits the length of the game, it's be too high level for D&D but works in a video game, and is the level the whole game is made. for 20th level which as you say7, it's still 20th level 5e didn't change that, having characters go to it IN THIS STORY, in this game? wouldn't work.
You'd need to
double the amount of acts, the scope, the scale and the stakes.
Is anyone going to pretend they got to 12th level half way through the game and spent the rest fiddling their thumbs against enemies more powerful than they could handle? 12th is good for this game and is what this game is built for.
MAYBE 20 level cap for the next adventure Tav & the crew gets up to DLC set after the events where they, idk steal a mindflayer ship and get lost in the planes? Go up against Withers because they finally worked out who he is? but not this game.
We aint godkilling super veteran heroes who rule nations in our off days and have had more adventures we can count, we're plucky underdogs with a secret weapon and an impossible mission, that for most will require the ultimate
sacrifice to pull off!
!2 is already kind of high for that, but it works because the game is built for it.