The only way to stay as true as possible to 5e is turn based. Especially, as you could see in the PAX gameplay trailer, that they use the granularity of action and bonus action and movement. Like in tabletop.
The only way you can do this is turn based, there is by logic no way you could reasonable use Realtime with or w/o pause.
And nice, if you can do this at your table anytime you desire, not everybody has a group available, whenever he wants to play real D&D.
I see turn base as a necessary "flaw" in design of tabletop as all players cannot act in the same time unless we have some kind of LARD/Matrix VR/Holodeck/etc... where AI can take all that information simultaneously. So I would not port that "flaw" to CRPG where it is not essential.
With BG3 you have move action bonus action and reaction. As you correctly pointed out the situation "all happens simultaneously" cannot be portrayed correctly in 5e Table top. The same goes for BG3 had they chosen to use RT.
Nope you cannot. Think attack by a mob. According to the 5e rules you got the option to use your reaction to cast shield e.g. as a wizard. So tell me pray, how should this work out? Hit a button on your keyboard as fast as you can to get your reaction in? With 4 PCs or so?Why not? You can do it perfectly.