When hardware companies sell add-on software subscriptions for their hardware products, this provides an incentive for them to maintain good support even for older hardware.

Apple is doing this successfully with the iPhone.

Apple wants a big install base and thus they keep supporting old devices.

I am thinking of replicating this model for the e-paper smart displays that I am making.

Though it must be very clear that any feature available for free at purchase must remain free forever. Otherwise you lose the trust of your customers and that’s the beginning of the end for any company.