I have a 2012 non-retina MBP with SSD and RAM upgrades. It was going fine until last week when I kept getting the spinning beachball. I deleted lots of stuff to free up SSD space to make it go away but it kept beach-balling all the time. I then did a Time Machine restore to a previous week when it wasn't beach-balling, but it kept happening anyway.
I need my MBP for my work and can't deal with the constant beach-balling, so last Saturday I got the 13 inch 2020 MBP, 2.0Ghz CPU, 16GB RAM, 1TB SSD and new Magic Keyboard. I got almost exactly 8 full years out of my 2012 MBP and that's a pretty good run. This new MBP is great and I am going to enjoy it while I have it.
Now that Apple has confirmed this move to ARM, I doubt I'll get another 8 years out of it like my 2012 MBP, but it's not the end of the world. Like other people have stated, the last PPC OSX Leopard was released 4 years after the announcement of the PPC to Intel transition, and got two more years of security updates, so that's 6 years.
The new OSes aren't always that much better. I was previously on 10.12 Sierra and skipped 10.13 High Sierra entirely and had waited until 10.14.5 or so to upgrade to Mojave. I wasn't really missing that much.
I have a good friend who pointed out that since there's millions of Intel Macs out there, he thinks they'll be supported for 5-7 years by Apple, which made sense and made me feel better. Plus Tim Cook said they'll support Intel Macs for years and years.
If I can get 6-7 good years of Apple support out of my new MBP, that's pretty good and I'll be happy enough with that. You can't expect everything to last 7+ years.