If someone (say FlightSafety and NetJets) want 25 new sims, don't you think the sim builders are interested in doing business? On top of that, new sims have to be built when new models are introduced and when in-production aircraft fleets grow. FSI-CMH is getting a new Encore+ sim that's slated to be in use by July 2009.
I can't imagine all the work it takes to build one of those things. When you build in the fact that FAA probably needs to bless a couple hundred times along the way....the lead time really adds up.