If AMD released a desktop class ARM processor at that time, what software would it have run?

Apple had already switched cpus in Macs twice, it's not surprising that they could do it again, but would they have switched from Intel x86 to AMD ARM when they never used any AMD x86? Seems unlikely.

Focusing on a product that would sell on day one rather than one that would need years to build sales makes sense for a company that was struggling for relevance and continued operations.