Reportedly, Apple is not planning to update its iMac line for a while yet. Instead, a next-generation 24-inch model will not launch for at least 18 months, albeit with an Apple M3 chipset. Meanwhile, the iMac Pro is said to be a way off from being released and will not arrive 'anytime soon'.
https://www.notebookcheck.net/Apple-analyst-delivers-expectations-for-next-generation-iMac-and-iMac-Pro.615392.0.html
My guess is that is too early to speculate or understand a pattern in Apple plans for their new AS macs.
While the Mac Book Air hasn't been updated in 18mo, there has not been a new M-chip released.
Apple have had no problems updating its iPhones and iPads with new chips in the past as they get them, the exception have been the high performance variant.
Updating the iMac to M2, in the case they update the Mac mini too, will cost next to nothing to Apple.
In the past updating to a new Intel chip eated Apple profits,