I don't think Routh was bad, but Routh didn't really bring anything new or great to the role. I know I'm projecting the wrong things here, but whenever I think of Routh, I think of the Superman who had the ugly-as-sin costume, accidentally playing a creepy Superman, in a misguided sequel directed by a guy accused multiple times of being a sex offender, opposite another guy repeatedly accused of being a sex offender, alongside the worst Lois Lane actress we ever had. Maybe none of the movie's problems are Routh's fault, but just like Cavill, when your portrayal has that much baggage I'm not going to get attached to the actor.
Canceling the Flash movie would be unfair to a lot of people who have nothing to do with Miller's being a problematic person. That said, I look forward to when skilled VFX artists cancel Miller by replacing each seen with the Flash with Grant Gustin.
EDIT: Oh geez, more Ezra Miller legal stuff.