Why in the world would they demolish that building? They've pretty much added a cavalcade of extra decorations, and etc. It'll pretty much be a landmark when The Simpsons ends. I doubt they'd ever demolish this building.
I could see them gutting the building, but not demolishing it. That would be an abundance of equity down the drain.
If Disney filled the 20K lagoon, Universal could knock this building down.
They could always put The Fast and the Furious in like they had considered at one time. That franchise has legs and generic car chases can bring in people.
I would say 5-10 years.
I honestly loved BTTF. It is such a classic, and overall, great storyline. I like the simpsons, but i feel that its going to get old once Harry Potter comes around.