People tend to confuse software support with how much people keep their devices....
If Apple is supporting the Air 2 for 8 years (2014-2022), or even more, the M1 will get at least 10 years, if not more...
How much people keep their devices? I think we can make 2 categories
1- Non enthusiasts: they tend to keep them for as long as possible... sometimes beyond the supported lifetime... (some people still use the Air 1 or even older). In these cases battery is often the triggering factor. Some people have a battery swap others upgrade when the battery is bad since the $100 swap is generally not worth it on a very old device.
2- Enthusiasts: there are 2 sub-categories:
a. People than have only 1 iPad, or at least one main iPad. These people will upgrade much earlier than end of support. Some will upgrade every generations, others every 2 generations, but probably 4-5 is the maximum....
b. Multi-iPad people: these people have several main iPads (e.g. 12.9, 11, mini). They generally don't upgrade them all the the same time, so they may have the latest for one size and keep another size for many years.... (sometimes using battery swaps to prolong the life of the devices)