Currently I am using my old 2014 iMac retina (512GB SSD, 16 GB RAM) and a 2020 iMac with maxes out CPU and GPU, 64GB RAM and 4TB SSD.
In the daily work I feel no difference, in fact I was disappointed about the speed when first setting up the 2020 iMac. Sure, there are occasions where it is much fast, probably I am just not using them to much.
An entirely different case was switching from my 2010 iMac with HDD and 12GB RAM, this is barely unusable anymore.
I also have a 2008 MacBook Pro wit 8GB RAM, I changed the HDD to a SSD and installed Linux. I use it for testing purposes and it is still very much usable.
So I guess as long as you have enough RAM and a SSD you can use the Macs much longer than Apple provides updates. Quite sad for a company that mentions the environmental impact in every presentation...