Engine is irrelevant. There's nothing unique about Larian's engine that makes the game wholly distinct from any other game that could possibly be made... Heck, BG1/2 didn't use that engine in the first place to create the series.
Other engines can work to provide a game that feels like a BG game. The only thing it will lack is Larian's signature "Surfaces" addiction which was homebrewed into the game in the first place (And is as a result subject to much chagrin).
I mean, we see other CRPG games using their own, non-Larian, engines don't we? Kingmaker/WotR/Rogue Trader, Dragon Age, Pillars of Eternity, Spellforce... With many companies simply moving to the new Unreal Engine anyway.
It definitely isn’t impossible to create another RPG like that in a new/different engine but that would be a whole lot of work. I think you are under appreciating a lot of Larian’s unique features (heavily systemic nature of a game, with objects being highly interactable, singleplayer/coop support, AAA isometric etc, motential moddability that you won’t get with licensed engine). All the games you mentioned are far simpler games with less moving parts. No other RPG does what Larian does, and I suppose, like Bethesda, Larian games and engine they are built in are intrinsically linked. There is also a matter of staff that needs to be equipped to build an RPG of that scale. It took Larian 3 games to get here. No one is going to fill a void, definitely not soon, just as no one is in a position to make Elden Ring2-like thing aside from FromSoft.
