Why would they be so desperate for an excuse that they'd upend the entire Mac world just to invent one? The last Mac CPU transition worked like this: as soon as they thought they could do it without significant repercussions, they stopped supporting PowerPC Macs and didn't give any excuses or explanations.
Why would this time need to be any different? I suspect it'll take longer, because last time the outgoing pool of PPC Macs was so much smaller and felt obsolescent almost immediately. But when it's time for Intel support to go away, I expect it to play out the same way. They'll just announce that the next release of macOS is for Apple Silicon only, and won't bother rationalizing it.
Also this isn't even a decent excuse. There's no reason iOS couldn't be ported to Intel. As others have said, and as everyone on the technical side of things has known since forever, iOS/iPadOS are just forks of macOS to begin with. Code sharing happens all the time.