When the RTU expires, you lose the ownership and I don't think you would receive any residuals based on the value of the land or property. It is just done.
When they close the sales center there, that will be your clue that your ownership will soon be gone.
But being more serious, I can imagine that MVC won't want to give up the property either, so they will probably be trying to negotiate a new RTU period. Now whether that would have any benefit to current owners, I don't know. They might just start over selling RTU weeks to new owners, or they might throw the usage into the trust and sell more Abound points. I'm not quite sure how they would work though – not sure how they can add RTU to the trust since it isn't owned property. So if any of us are still around in 2061 – I probably won't be – it will be interesting to see what happens.
The other question might be, how badly will the property decline in the last decade or so if there isn't going to be a new RTU period? MVC would have little incentive to do anything more than the most essential of maintenance to fulfill their waning obligations to RTU owners. They might even decide to take the Marriott name off the property so that they didn't need to maintain brand standards.
Have any of the MVC RTU properties ever reached the end of their RTU periods? I don't think it's happened yet, right?