One of the issues with crowning a GOAT before they are, you know, The GOAT, is that the body of work is incomplete. Mahomes is now 3-2 in Super Bowls. Brady was 3-2 in Super Bowls, and Montana was still the GOAT, and people debated that Manning was better than Brady. It wasn't until the Second Renaissance that Brady closed it out. Once Brady won the Atlanta game, the debate was pretty much over. The Rams II was the cherry on top, and then the Tampa win was proof that he didn't need Bill anymore.
Mahomes can still be the GOAT, but it's waaaaaaaaaaaaaaaaaaaaaaaaay early to have those discussions. Let's wait until he's in his twilight to measure him up. He's the best in the game right now, but he's far from the rarefied air that Brady flies in.