A move to ARM on anything other than entry level products and the MacBook would be a big mistake, unless Apple intends to keep Intel for the Pro models. No way ARM can compete with the 65W+ Higher end Intel and AMD 6-8 core multi-threaded high end desktop chips.
As for an iMac refresh, using older chips is a big plus for Apple since it improves margins massively, and under Cook that is all they care about (even the new Mac Mini wasn’t given the latest gem stuff when it was shipping before the Mini came out). Could be related to the chip production shortages Intel is having due to their messed up/multi-year delayed transition from 14nm nodes to 10nm.
In the end the reality is that the Mac is now just a footnote that Apple staff remembers to do something about at 3PM every Friday afternoon.