I don't think "nothing at all" is on the table, unless the attractions they're closing cost a preposterous amount of money to run. I can't imagine that being the case for Primeval Whirl, although Rivers of Light might be rather expensive. Special effects (and possibly actors) costs money, after all. No word on Dinosaur closing as of yet, which is probably the most expensive attraction to run in that section of the park, albeit nothing compared to Flight of Passage or Everest. The rides are also still fairly popular, so I imagine there wouldn't be any net savings in keeping the rides running as-is. Removing them to make room for new additions is probably a more safe bet, although simulators and dark rides might be very likely indeed.Then again it could be another simulator or dark ride, or maybe nothing at all.
The other option that could lead to rides being scrapped without replacement is that something is broken and prohibitively expensive to fix (see the Everest yeti for instance). But I can't imagine repairing Primeval Whirl, which reportedly is broken, would be that expensive. It's a Reverchon in a parking lot; they could call up any park in the UK and get all the replacement parts they need.
However, for Stitch's Great Escape, an outright removal without replacement could actually be possible. It was never a well-thought-out ride, being a hasty reskin of the way too dark Alien Encounter attraction. It's not a very big building either, so it's not a location they could use for much else. And I think it has quite a bit of scope overlap with Monsters, Inc., which is literally next door. The timing is pretty strange, though, as a live action remake of Lilo and Stitch was just announced. Wouldn't that spark renewed interest in the character? Then again, it's reported to be released direct-to-streaming, and live action remakes don't have much staying power. I mean, raise your hand if you remember that they remade Lady and the Tramp a few years back. Alternatively, they could have bigger plans for Stitch somewhere else. Either way, I don't think many tears will be shed over the demise of Stitch's Great Escape.