Alas you have bought into the DIm language that some program passed by one Congress can not be undone by another Congress. What you along with the DIms and MSMediots call mandatory spending or entitlements are in fact promises.
Some promises are kept and some are not. One Congress can not bind another Congress to keep its promises. So far with the more popular of these promises Congresses have gone along with past Congresses.
If the economy does not start growing and generating more revenue, that will not necessarily continue forever. When two workers are supporting one retiree, the retiree necessarily will get only some fraction of what the workers produce, no matter what the retiree was promised.