What if making Diablo 4 turn based makes it better?
Such a scenario is so implausible and absurd that it made me chuckle.
The turn-based-only "I ❤️ slow combat" crew would obviously love that, but Diablo would fans would hate it and revolt. Rightly so too.
Not to mention that a turn-based game could never sell over 30 million copies like Diablo 3 did. A turn-based has not and never will come even close to such figures.
Bonus trivia: Diablo was originally a turn-based RPG, but the developers adopted a realtime combat system rather late in development.

To be honest I played very good games in TB and in RTwP but I´ve never been able to fathom why so many people think that TB games do not sell well.. I mean, many of the top-selling videogame RPG franchises are TB games. Dragon Quest franchise, a japanese TB games, sold over 76 millions world-wide, it doubles the figures of the witcher, diablo or Elder scrolls franchise. And I do not even want to tell you the sales of the Shin megami tensei Persona, Tales of, Dragon quest or final fantasy (the best sales went to the TB games of the franchise), all with more than 10 million sales.
TB RPG games always sold well, history-wise.
https://vgsales.fandom.com/wiki/Best_selling_RPG_gamesAnd I didn´t even refer until now to the second-best´ selling videogame franchise of all times (Of any category), they are turn-based games (Yeah, It´s Pokemon) with 346 million worldwide,
https://en.wikipedia.org/wiki/List_of_best-selling_video_game_franchises