The infinite horizon is a bad number because it tells us absolutely nothing. We do not and cannot have enough information about Social Security, about anything in 2500 to make any projection that has any meaning whatsoever. According to Star Trek, we can buy machines that replicate food for free by the mid-25th century. According to any number of bad Mad Max ripoffs, we'll be lucky if we aren't eating rats stewed in expired chicken broth while lame ass punk-wannabes ride around on motorcycles taunting us.
You can't make a projection without information, and we don't have information.
Then in the comments:
Post-apocalyptic biker-punks need to stop their partisan sniping and present their own plan for Social Security's future.
Somehow I don't think they have this much fun on the other side...