a used Mac will have an even shorter support runway than the currently sold macs. All the "older" ( i.e., have been replaced) Macs have a actively running Vintage/Obsolete countdown clock already running. The currently sold ones do not.
As for Apple trying to push folks off of Intel Macs. There is over 100M of them at this point. That is very substantially more inertia that Apple had to push aside on the previous transitions. It is very likely not going to be as quick as the previous iterations. Piled on top of the several 10's of millions more Macs Apple has to push aside there is also the factor that in general the majority of classic PC form factor owners are holding onto systems for longer periods of time. The upgrade cycles are longer. ( partially because folks throw so much money at iterating faster on fancy smartphones and other tech).
If Apple pushes toooooo hard the number of folks switching back to Windows will go up to a bit higher than those jumping over( Apple probably looses some services revenues if the primary host system drops out of the Mac ecosystem because have pissed of the user. So it is a case that Apple now has the small revenue stream attached to that 2020 Mac that they did not have with the 2004-2006 Macs. ). On average Mac owners switch incrementally faster than generic average Windows PC users, but the trend line is generally up longer even inside the Mac ecosystem. Unless Apple radically drops the average system selling price on a Mac , the demand probably isn't going to very dramatically increase. It will be more than just a couple of years before Apple sells 100M ARM SoC Macs.
Apple probably is not going to give the last Intel Macs the absolute maximum support windows, but also not likely to grossly cut them off prematurely when still making money with them either.
This screed bears very little resemblance to the Apple whose products I have used since 1989. This may be the way that Microsoft does things, or IBM, or Intel, or many other large corporations with large installed bases. You see many of these companies hold on to legacy products forever.
Since 1976, I have seen Apple introduce new products, sucky products, spectacular products. I have seen Apple introduce products that literally defined their product category. I have seen Apple withdraw products that were not doing so well. I have also seen Apple withdraw category defining products that continued to doing very well in the marketplace.
Steve Jobs made some comments that defined Apple's product philosophy for good or bad. One of his most memorable was "skate where the puck will be." Yes, Apple follows some markets. But the philosophy that defines Apple is to create markets that did not exist before or to dramatically redefine markets in ways that were not thought possible. I have never seen Apple try to force people to switch to anything. When Apple makes a switch, the customer may go along or go elsewhere. It is the customer's choice. It can be very frustrating when Apple ditches a product that I rely on. As much as I may want to replace my old AirPort wireless router, Apple thinks that its resources are better spent elsewhere. You want a LaserWriter? So what if they defined the category. There is no profit in them. HP can have the sale.
Apple's philosophy has helped build it it a $2 trillion company. It reminds me of Olympic gold medalist runner Michael Williams whose bolt upright running style set him apart from every other running. He was asked "Why don't you lean forward when you run?" Williams responded: "If I ran like everyone else, then I would finish with everyone else."
