Thank god they didn't. It would have been such a waste. The SE helix has been an under-utilised dead spot right at the front of the park for almost 2 decades. This absolutely is the right space for a mediocre flat ride of this size.
Yea, Doomsday sucked and was a bad choice for the park, but that plot of land was well-themed when it opened and it deserves a better ride than a Surf Rider. If anything, they've missed an opportunity to update SE's story, theme the Flash to Villains and put a lot of that theming at the front of the park, all the way up to Rivals. But that would have required them to commit to replacing DD with something better, and re-theming the area - which is expensive.
If they kept the theme, there are plenty of reliable, relatively inexpensive classics that could fit there - like a Top Scan (just an example).
But I'd personally like to see a re-theme. DC is basically dead now as a 'movie' franchise. Dune and Madmax would be my picks, and they both lend themselves to immersive theming of an outdoor thrill flat, with an indoor simulator (like batman) in the show stage.