I've been patiently waiting for Android 7.1 to come to my Galaxy S7. If Google could offer me the latest OS upgrade now for a modest fee I would jump at the chance. I think plenty of other people would too.
It would give them a chance to monetise their investment, it would allow them to wrest control back from the phone companies and manufacturers with their added dross, and would let Google effectively dictate which devices were worthy of their effort, potentially steering purchasers towards vendors they like the most. It would likely distort the market by concentrating on premium devices but that would also go some way to solving the issue of devices lagging behind on updates ('our customers get them fast and first').
I'm not sure that this would be a good thing, you understand, but I can see that it could be done. I'd guess that it's politics that stops it, not any technical reason.