Given that x86 still has an advantage on servers makes sense for them to push that for then time being. When the Arm ecosystem is fully established I can't imagine it would be that hard to introduce a new Arm CPU using the innovation they've brought to x86 (chiplets etc).
ISA changes require a long term investment and building up an ecosystem. Which were out of scope for AMD at the time.
I think the market has changed somewhat, and they don't have to do all the heavy lifting. Would be interesting to see what happens there.