As opposed to what? There's no other good way to control multiple characters simultaneously outside these two systems.
Even controlling one person, if they were going to do an ARPG, they would've continued the Dark Alliance series. A FPP game is waaaaay out of Larian's wheelhouse and the engine they use probably won't support it. A TPP OTS D&D game is already a thing and it's completely dead.
So BG3 will be a battle royale roguelite MOBA card game? Lets get real, it's going to be RTwP or TB isometric top down.
Larian has now over 200 all over the world employees that are more than enough to make a third-person game. They are no longer a cute Belgian indie studio. And Swen loves to talk about his dream game which will be AAA game, so sooner or later they will be abandoning the old perspective.
Old perspective? o.O Action 3rd person games are as old as videogames themselves, from platform adventures to beat-em ups to shoot-em ups. RTwP is in fact a more modern mechanic in videogames.