Big Sur will be EOLed in November 2022. If they really cut off v12 from late-2015, they will have provided 7 years of support by then. Slightly earlier than their usual 8-10 years cycles, but arguably warranted by the significant technological change of T2.

I don't think they'll push hard on replacement; after a bit of (warranted) skepticism, the m1 is selling itself. Besides, what really matters to them is the yearly iPhone cycle; as long as that keeps spinning, they can be generous with the rest of the ecosystem.