For the $1800 - $2300 27" Intel iMacs, it rather depends what you think is a worthwhile upgrade. The way I'd look at it is that the old $1800 i5 27" iMac was roughly equivalent, CPU wise, to the $1100 i5 Mini so it "felt like" you were getting the screen, speakers, webcam, keyboard and mouse for about $700. That deal is over.
Even if you decide that a base $700 M1 Mini + studio display is a "worthy upgrade" from an i5 iMac, you're paying $2500 (don't forget about $200 for a keyboard and mouse/trackpad). It probably is "worthy" CPU wise - but while the GPU thrashes the Intel Integrated in the old Mini it might not quite be so impressive c.f. the old iMac discrete graphics. You're also a bit swings-and-roundabouts on I/O and monitor support - fewer external displays, fewer USB 3 ports vs. better TB3 bandwidth on the M1, and the 10GB Ethernet option.
If you're looking at a top-end (but non-pro) iMac then the Studio Max plus Studio Display is pretty much the same as you used to pay for an iMac with the best CPU, best GPU and 32GB of RAM. Likewise, the Studio Ultra + Studio Display (or cheaper alternative) seems like a reasonable substitute for the iMac Pro (esp. with the 20 CPU core Ultra vs. the 18 core Xeon option that used to cost $7000+)
The downer is that the display panel is pretty much the same as you got 4 years ago, too (just 100 nuts brighter which may not even be helpful)....
What seems to be really missing are (a) the lowest-end 5k iMac and (b) a half-way house between the Mini and the Studio Max (to pair with either the Studio display or a third party unit). That would have been a M1 Pro in either Mini or Studio style. Personally, if that's what you needed, I'd pin my hopes on a (regular) M2 Mini appearing which is likely to offer better bangs-per-buck than the M1 Pro until/unless a M2 Pro appears.
I think the problem with the future of the iMac is that while some people like the all-in-one-ness of the iMac, others prefer the flexibility to chose your own displays offered by the Mini and Studio - so you'd have the Mini, Studio and iMac competing for the mid-range desktop market, and Apple don't like competing with themselves.