As a fan of the old games, I think that the amount of adaptation and hammering necessary to shoehorn BG1 and 2 into BG3's engine would produce something quite inferior to the originals. BG3 sometimes cares too much about making players feel like they are playing a virtual tabletop game, with its dice rolling animations and turn-based only combat, which isn't a philosophy present in 1 and 2. In the old games the underlying rules system is more like a implementation detail than something you have to interface with during gameplay. The lack of day/night cycle would also be a big no for a remake, especially in BG2.
