Rumba has been a shell of itself for basically a decade now. Expensive to run and likely not well received meant it was always on borrowed time. I believe there's been more than one conversation over the last few years about closing it down.
The situation is very different to the likes of Slammer and Loggers Leap. Slammer closed due to spiralling maintenance costs and limited support from S&S. They literally tried everything they could, but they had to give up. Loggers required more money to fix up than they had available so they genuinely closed it with a plan to reopen, but then costs spiralled.
Rumba is a clear and conscious choice of "we don't want to run this any more".
The big thing is there's no reference to a replacement planned. It's just been closed. Sure, it can be hidden reasonably well, but it's closed and nothing looks to be on the horizon.
The park has a lot of water rides,
Not any more, really. They've got Tidal Wave, which as an oversoaker and only worth it on the hottest of days, Storm Surge, which is a gentle splash, and Depth Charge, which is just a slide. Before the closure of Loggers, they had a good, varied water ride line up. Now it feels severely lacking.
And the point of Tidal Wave brings me to my next point: some people say we don't have the climate for water rides. They are only seasonal rides. Yet that's never stopped water rides here being open March-November in the past. Tidal Wave, which dumps 2.7 bathtubs of water on each person, is open all season. Why can't we run a rapids or a log flume?
As for the future. I think a new, modern, water ride should take its place. They can have restraints to help ease the Merlin H&S concerns. They have adaptability with how wet you get. And there's lots of options out there.
-Something like Amazonia from Bellewaerde.
-An Intamin flume (where Intamin offer all crazy extras now)
-Mack Rocking Boat
-Mack Water Coaster
Plenty of other options too.
Obviously they could just do the coaster route too.
Something that people have noted is 2029 is the park's 50th anniversary, so suggestions they could do something big then. And obviously with Universal coming in 2031, it'd be ideal to have something big and new around then. But I hope we don't have to wait that long for Rumba's plot to be used...