Just gonna say it. BG3 is based on D&D 5e, which is, hear me out here, also a turn-based game. So it is entirely in keeping with the rules of the source material. If they were making almost any other sort of game, you might have a point. But 'Turns' have been part of D&D from the beginning.
Have you ever played DOS or DOS II? Why do you think early on and maybe even now, BG3 was/is refereed to as DOS III?
My original comment has nothing to do with how D&D is played, its a table top game and each player gets their turn, fine I get it.
This is about making/creating a World which a single player or one with companions can explore, do what ever is the story line, etc...
In a open world type environment such as the WItcher 3 or Cyberpunk 2077, or even Fallout 3/4 or, or dare I say it, the grand daddy of them all, Skyrim, which people to this day cant stop playing!
I'm not a game developer but I would think it would not take longer to make a open world single player RPG than its taking to create BG3, yes, maybe they would need to use a different engine and learn how to use it effectively which would take time, but they also don't need to make it a 100hour + game, maybe start small, make a 50 hour game, to get your feet wet, see how its accepted and work from there.
Also, the idea of "spell slots" rubs me the wrong way, if I'm a Wizard/Sorcerer, I learn my spells, have then reedy and the only thing I need to worry about is my MANA (or some other unseen magic force), MANA which can replenish over time and perhaps faster if i camp or sleep at an Inn or eat some food, etc....