I agree that the manufacturer or carrier doesn't owe you updates indefinitely, but they definitely should give you all the resources you need to develop these updates yourself if you wanted to.
The landscape can change. Maybe there's going to be a huge open-source project that will make these updates. Maybe someone will figure out a way to automatically handle a large number of hardware configurations at scale (a powerful enough hardware abstraction layer should be able to deal with it). Maybe someone will make a business out of it, providing security updates at a reasonable cost. Either way, there shouldn't be any artificial hurdles against the customer developing and running their own firmware on hardware they paid money for.
I would really like to hack on a tablet I have that's 5 years old, but I can't because the firmware and drivers are both closed source.
I think when I buy a device, I should get the source code to it once the manufacturer decides it's not worth it for them to continue patching the software for the device I've bought.