It doesn't have to be that expensive to continue supporting old devices. If all your devices use the same software, and new versions don't add new hardware requirements it wouldn't be that hard to continue supporting old hardware.
But in the current ecosystem every device has its own medley of custom firmware and software that gets abandoned when the maker stops selling that version, and the makers are incentivized to stop updates to drive customers to buy new devices.
But in the current ecosystem every device has its own medley of custom firmware and software that gets abandoned when the maker stops selling that version, and the makers are incentivized to stop updates to drive customers to buy new devices.