The role that the Haunted House building has played in the development of Alton Towers history cannot be overstated. The 1992 investments really did move Alton Towers forwards from a park with some nicely themed individual attractions here and there to a park that was serious about becoming one of the top 5 parks in Europe by the turn of the 21st century.
There is no reason why Merlin couldn't inject some cash in the next few years in the ride, restoring it to former glories, re-marketing it and making it superb once again. The ride system needs a full PLC upgrade, which Mack could come in to do, and the scenes need thorough work and, in some cases, rebuilding. This would take £5m+ though.
I worry, though, that with the push to IPs, that the ride could be bulldozed by 2025 and replaced by something current. Of course, as with all IPs, so much budget goes on the rights - that it leaves much less for the actual ride. I would really hate to see the building and ride system removed.