To be honest whatever happens to ghost train, flattening the building after 10-15 years of operation would be ridiculous quite frankly. Can see the building being repurposed for a complete new ride system one day. Even the possibility of combining it with the area of the walking dead when that goes.
All that being said, the walking dead ride building also has solar panels adorning it, I don’t foresee that being there for another 10 years, however I’ll caveat all of that by saying this is Merlin, and there are far more areas requiring attention before removal and replacement of rides, including new areas ready for development.