I think compatibility to IBM PC has lots of hidden costs. One is that it seems to make Intel much less nimble at producing SoCs with features that are specific

I think compatibility to IBM PC has lots of hidden costs. One is that it seems to make Intel much less nimble at producing SoCs with features that are specific for each usecase. It's not just the ISA, it's basically the whole ecosystem around PC that's not well adapted. That's why we don't have Dell, HP and Intel phones and hardly any Microsoft ones today. x86 may not be the only reason, but from what I can see it's at least one of the major ones.