The BIGGEST issue is that is wrong, evil, and outright thievery.
That it is unsustainable is a side issue.
If it were, as you say, financially feasible, then it would be ok? IF that were the only issue, or the MOST important issue, then it would be ok?
Unfeasible or not, sustainable or not, theft from one generation to another is wrong. And that is the bedrock basis for why SocSec and Medicare needs to go away.
Its no more evil then the other taxes taken at gun point.