The trustees of the Social Security Administration have traditionally used demographic, economic, and fiscal models that end after 75 years. The idea is that there simply is no way to accurately predict the future beyond that. (Aside: can't we fix the long-term problems of SocSec by making _demographic_ changes? Exempli gratia: increase immigration.) The current administration made waves this year by integrating these numbers out to infinity to overstate the crisis.
We (society as a whole) would be irresponsible if we did NOT plan for the future---at least 75 years out. On the other hand it would be a waste of time to plan beyond 75 years.