Just reading through the suggestions pages for DOS 1 & 2. Interestingly, I'm struggling to find any requests there at all for increasing the party size. Almost as if it's not critical to the game's enjoyment level in this engine.
My 2 cents : it's not the same game, and not the same system. In DOS and DOS2, classes are "fluid" : you can chose your skills the way you want.
Also... D:OS2 had less companions. We don't know how many it's going to be in the final game, but it's at the very least 7. Not a huge difference at this minimum, but BG3 is supposed to have more companion interactions, so the combinations of companions will matter much more, and a bigger party size is going to enable us to see more of those. In terms of replayability... there's no need to artificially inflate it, the game is going to have insane replayability anyway.