First of all see the "impoundment power" and the Impoundment Control Act. It's not clear what the Constitution says on this matter, nor have the courts pronounced themselves on this. In the 90s the SCOTUS said that Congress could not give the President an explicit line item veto power, but impoundment is not the same thing.
Second, in extremis, it has got to be the case that the President can refuse to spend allocated moneys. The President must never be able to spend unallocated funds, but Congress can't make money appear out of nowhere. In extremis the dollar could stop being the world's reserve currency, and in extremis the U.S. could end up in a situation like Argentina's. Obviously there has to be a limit. I'm not saying we're there. But I am saying that the courts should thing ten times before saying that the President cannot impound spending.
Besides, consider all the constitutional provisions that we no longer adhere to. For example the Constitution says that Congress must set the value of the currency, but Congress does no such thing. For another the Constitution says that all debt issuance must be approved by Congress, but Congress merely sets a ceiling for debt issuance rather than approving each issuance. Some things -not many- the Constitution got wrong in ways that nobody even bothered to amend nor ever will.