Since the Intel-baed Macs introduction in 2008 or so, there have been millions of these devices sold and Apple will continue to provide OS updates and support for the forseeable future. If you look at their current plan with the currently released Catalina, it basically marked any Mac prior to 2012 as vintage and unsupported, so that's 8 years of Macs.
Big Sur will likely not move that needle yet, but I can imagine one or two more releases and the line might move to 2015 as the newest supported Macs for continued support.
With that pattern, I'd expect the 2018-2020 Mac models to be deemed vintage around 2030 or so.
I recently purchased a 2018 Mac Mini and totally expect to keep the OS up to date for the next decade at which point in time, I'll be ready for a new Mac.
That was the pattern of my last Mac a 2009 iMac that lasted me until this Mac Mini replaced it. That machine got stuck at High Sierra and still worked well enough, but with my development needs, it didn't fit the budget. However, as a generic web browsing, video streaming computer, it's still working for the individual who bought it from me.