I'm sorry, but what exactly are you basing these assumptions on? lmao Arkham Knight still looks amazing to this day and that was on a modified UE 3, while their next title will be running on UE 4. Also, Rocksteady's titles have been praised for their gameplay. My only concern is for the story, but luckily, they do have some very good writers on staff (senior writers who have worked on Mafia 3 & Horizon Zero Dawn).
Before you mention it, yes, I'm aware that they've recently hired a Destiny 2 writer.