RX 7800 XT was supported for 15 months before being dropped. Significantly less than 5 years.

True but business hardware (and home for that matter) often goes on 3-5 year cycles though. At 5 years it's kinda expected hardware will get replaced.

It doesn’t work for the first three years, so it’s two years in practice.

And that's if you're buying bleeding edge new hardware. If you can't afford to buy it the year of release the window is even smaller.