My original plan was to hold out for the M-chip iMac or move to an M1 Mac mini. However, went ahead with picking up a 2020 iMac 27" 8/512GB (stock config) from Costco on sale (not quite as low as their November sale, but close enough).
My Late-2015 17,1 iMac is running OK now, but given the ongoing firmware issue (see the other thread about 17,1, SSDs, and blocked updates), it just edging out of extended warranty coverage, and given that it was down for over 4 weeks for the last logic board repair (the last under the extended warranty coverage), I felt it was worth moving forward with replacement.
It feels odd buying a "new" computer and having it look exactly the same as the previous one. But I know there are improvements under the hood that will be welcome (better FaceTime camera, USB-C, Bluetooth 5, etc.) even if they aren't readily apparent. Upgradable RAM is also something that I don't expect to see on the next iMac. And having this new machine covered effectively for 5 years under a warranty (AppleCare plus 2 additional years by Citi) is reassuring.
The M1 mini was tempting, but after researching monitors, I realized how much I liked the iMac's 5K display and how simple it was since all just worked. The only other 5K display I found was the LG at $1,300. I thought about getting a 4K 27" Dell Ultrasharp, but read about why 27" 4K displays are not necessarily ideal.
I'm sure my next Mac will be an M-something, but I'm confident that my "mature" Intel-based machine will keep me satisfied for another several years.
Edited to add: I was a bit surprised that my 20,1 iMac came pre-loaded with Catalina (Costco reportedly just got these, but maybe old stock?), so getting it up and running is taking longer than I thought.