Yeah agreed I would have thought they would remove the ride hardware and replace with something else. However it will be a challenge as I think that building was specifically designed to hold the current ride system, (I.e. the shape of the building).
Regarding Walking Dead, I would like to see them rip out the coaster (its 30 years old next year!) and putting another dark ride in there. Would think its possible, and would certainly save on building a new structure if they were to go down this route. Don't think Thorpe needs another coaster yet, and if it they were to build another coaster I would prioritise the island next to Swarm or replacing Colossus.
Regarding the island next to Swarm, I think a good way to do it would be to put a flat ride there first, and build the necessary infrastructure to connect the island as part of that project. They could then invest more money into a decent sized coaster there rather than having to split it with infrastructure as well (just a thought). I even had the thought that they could move Rattlesnake over from Chessington and putting it on the island as a supporting coaster, as I don't think it fits the Chessington line-up that well anymore (particularly with the 1.4m height restriction).