Agree -Any computer launched in 2017 is about 5 years old now in terms of components. It doesn't become 4 years old just because you waited 1 year and bought it in 2018.6 years of OS updates seems pretty good to me. The difference in the A chip launched this fall (A16), is drastically more powerful than what would be dropped (A10). It’s got to happen at some point, and that seems very fair.
Also, I would not wait years to buy a device. I would always buy the current one. If you’re going to wait years to buy, it’s well worth the investment to buy current (like a MacBook Air upgraded getting the M2 Air just released). It is not worth getting M1 or getting previous Intel models. You’re that much closer to obsolescence from the start.
And there's not set number of updates that you are guaranteed even if Apple is notorious for having supported some devices for many years.