Larian want to create a modern take of a old-school RPG IP with modern graphics and modern, streamlined game design principles. Not so hard to understand to be honest.
what are you talking about? DnD5e (wich that game is supposed to be based on) is from 2015. The rule set is pretty modern. BG1 and BG2 were based on AD&D (alias 2nd edition) and yes those rules were very old. And bad tbh.
I would also not want to have that back. But its not the issue that BG3 is not based on 2nd edition DnD, is it?
And its very much possible to create a modern looking good graphic RPG that for the most part follows the mechanics of a modern pen and paper. see pathfinder kingmaker.