Monterey supports the 2013 Mac Pro and 2014 Mac Mini, so if we can expect only 8 years of actual OS support, we'll probably see 8 more years of OS updates for stuff like the 2020 Intel iMac and Intel Macbook Air & Pro. That's a long while before this becomes a possibility.
As a particularly relevant counter example, the last PowerPC Macs were sold until August 2006. Mac OS X Leopard came out a year later in October 2007 and received its last patch (10.5.8) in August 2009. That's a lot less than 8 years.
This is why when the first M1 Macs were announced I immediately sold my 16" i9 MacBook Pro and bought an M1 Air for much less than I sold the Intel machine for.
Yeah while Leopard remained supported with some security updates, you could forget about all the OS update goodness which was still significant back then.
I had a PowerBook and it was disappointing how quickly it aged because of this.
https://support.apple.com/en-us/HT212551