I don't know how going from RtWP to turn-based, removing day/night cycle or limiting exploration, among other things is appealing to large markets. In fact the trend is the opposite where many franchises (JRPGs and cRPGs) went from turn-based to action. RtWP is the midpoint between them and it seems more logic to improve it or create a new system that would be more dynamic. A game with a technology from 20 years ago was able to represent a more believable world.
I agree that at this point they will not change the core mechanics, but it looks they decided to play safe instead of pushing the franchise and the RPG genre forward. It would made more sense to start from where BG2 left instead of starting from where DOS 2 left (which had much lower standards).
It was very easy to implement AD&D2 into RtwP because that was a much simpler game. I don't see how you can adapt 5e faithfully into a RtwP game. You would have to cut out so much. Reactions, bonus actions, all would become normal abilities, and then how would you balance all that? Juggling cooldown timers? Yuck. D:OS2 and XCom have proven that turn based tactical games have a market, so it isn't such a big risk. Playing it safe? Sure? Larian are making this game, and they should play to their strengths and do what they do best. That will make the best product possible. And I don't think there is as much hunger for RtwP as you do. Pillars of Eternity sputtered out and died in the second installment. I have never met anybody that says they like the gameplay for Dragon Age: Inquisition. Among those I know that do like it, it is the world, the lore, and the characters which draw them.
Anyway, If this game is successful, hopefully there will be a lot more D&D games to come, spanning a variety of genres and with different budgets. Despite owning the license to such a prolific and influential game as D&D, WotC has been pretty shy about engaging in the video game market. Now they are breaking out a bit. I don't know about Dark Alliance and their GoPro trailer, also, Drizzt . . . Blehg. But If WotC are feeling more comfortable getting their IP out there then maybe you will see the RtwP game of your dreams.
