Larian may be best at that particular combat system. But that does not make that combat system the best system. Two separate issues.
Never said otherwise. Some things work better for other things. Strategy games work better with TB, hack'n'slash games are better in real time, same as fighter games. Roleplaying games kind of fall into several niches. Action-adventure, strategy, third-person shooter, first-person simulator and any other number of subgenres.
D&D is a classic rpg with a certain degree of strategy involved. How 5E works is entirely dependant on the turn-based system to take into account the action, bonus action, reaction and movement mechanics.
In order to make a game based on D&D 5E as its base it would almost have to be TB for it to even work.
I'm not sure they will even be able to do 5e TB exactly. My understanding ( not a PnP player since 2e ) is that you can have 1 reaction per turn, but there could be many opportunities to take that one reaction. So the game would effectively need to offer each player a reaction every time one became possible, until the reaction is taken. That would make an already slow combat experience even slower.