Nowadays the proof of resolution of singularities in characteristic zero is considered something you can teach in an intro algebraic geometry course, though. The concepts have been absorbed and are now much better understood. 4CT is very different because so much of it is exhaustive case analysis; you can understand the high-level ideas of the proof as a bright undergraduate, but you still can’t check the cases by hand
Abhyankar and others spent years trying to find an easier proof. I’m not an algebraic geometer and I don’t know the state of things now. I was under the impression that on the level of Ideals, Varieties, and Algorithms one can introduce the concept and do some calculations but not present a proof of the theorem.
But the point is that pre-AI it was already the case that famous results were published that very few could understand or digest. I think it is reasonable to expect that we will soon be at a point that Lean says a theorem is correct but no human can or will ever understand the proof.
What if Lean verifies Mochizuki’s proof of the ABC conjecture. Do we disregard it becuase no other mathematician understands the proof?