So put that to 5nm. Either you've partly closed the large efficiency gap, or you've closed the significant performance gap, but in neither case will you come close to closing it entirely. So you're either a little less slower, but still a lot less efficient, or a lot slower, but a little less efficient.
Yeah, maybe they'll do some amazing things on the core that'll overcome all of this. But right now the Firestorm core has a massive advantage. AMD can go to Zen4 and 5nm and maybe they'll close the gap, but Apple won't be sitting still.
Apple entered the desktop space and brought the most efficient core (by _far_), hilariously offering the best per core performance without going to magnitudes higher thermal profiles. It's pretty amazing. So now we're into comparing it to hypothetic, mythical alternatives from competitors in the future.
Sidenote: Apple has had blazing cores for a few years now, and every denier will claim it's just some big cache or some other absurd simplification. If that were the case, everyone would just copy them.
And that's before considering the density improvement that came along with it (which is also substantial - TSMC's N5 is up to 1.8x the density of N7). Which is why I mentioned Zen 3 & Zen 4, because you don't make the same chip across a shrink. You use the extra budget to do things