Larian doesn't think this is what will sell copies of the game; they know DOS2 sold millions of copies, so the closer to BG 3 is to that, the more they anticipate they will repeat those results.
(For the record I agree they should try and remain closer to the source material for rules; HP bloat makes caster's feel awful to play and demonstrates a fundamental mistrust and misunderstanding of 5e)
The game will certainly have different levels of difficulty. The current difficulty level is hard (or very close to it), on lower difficulty levels HP will certainly be lower, so it shouldn't be a problem.
