Wait, so congress can throw away promises at its whims that was supposed to last until perpetuity? And SC will be fine with that as long as promise was thrown away explicitly?
And yes, the SC should be fine with whatever congress does as long as laws are obeyed in the legislative process itself.
This came up in the Brexit controversies when the government tried to mandate an exit date as being immutable by future legislation, but the Speaker wouldn't have it. In that case some Brexiteers cried foul that it was overreach by the Speaker, but of course such a weapon could equally be used to make our membership of the EU irrevocable. It's a sound principle.