You could not just slap real-time combat on the Original Sin games, it would be a completely unplayable clusterfuck, given that a single enemy can deal a lot of damage to a character on one turn alone. You think the complaints about "everything is always on fire" are bad now, just see how it would work in real time.
This is not something you can just slap on as "an option" and call it a day. The demands of the different styles require different approaches in design.
Eh, that's pretty much Kingmaker and apparently people love it.