- Apple has already transitioned iMacs, MacBook Airs, and MacBook Pros to Apple Silicon.
- Only the Mac Pro hasn't moved to Apple Silicon yet. (Also, 16" MacBook Pro from 2019 and 27"+ iMac from 2020, if you count those separately.)
- Assuming that Apple will no longer introduce any new Intel Macs anymore, the last new models that were released will be the MacBookPro16,x and iMac20,x, both from 2020.
- I think it's a pretty safe assumption that some time between now and the end of the year, Apple will replace the 16" MacBook Pros and 27" iMacs with Apple Silicon models.
- My guess is that the MacPro7,1 from 2019 will be replaced early 2022 and that will be the last Intel Mac to be discontinued.
- If the above guesstimations are correct, 3-5 years of additional support will mean that all the 2020 models and 2019 Mac Pro will gets support up to 2025-2027. That means 2019 Mac Pro and 2020 models will have received official support 6-8 years after their introduction. I think that's fair and in line with how long they've historically supported most models.
Supporting them any further than that will not make sense regardless of how much money the company makes. It would be fiduciarily irresponsible of management towards shareholders.
Edit:
When I say 3-5 years of support, I mean 3 years of new macOS releases + 2 years of security updates.