Earlier quoted context omitted.
Updating old products is expensive, especially as their hardware falls further behind. Unless manufacturers adopt some kind of subscription model to justify the cost of continued updates, which would likely be very unpopular, I don’t see how they can realistically keep older devices supported indefinitely.
Vendors do not have to support their product indefinitely. They have to design their products in a way that they do not become useless after support ends. E.g. by opening up the bootloader and letting users install custom software.
If limited to the bootloader, such as allowing users to run their own operating system, then that is definitely a possibility, with the exception of firmware running on the device itself. But I doubt that is what most users here are expecting, and in my opinion, it wouldn't provide much value to the vast majority of customers.