>We've already had TVs which only started serving ads after a few months of use. What's stopping them from selling TVs which stop working if it hasn't been able to connect to the mothership for a few weeks?

Same thing that prevents your phone manufacturer from adding a firmware level backdoor that uploads all your nudes to the mothership 1 day after the warranty expires. At some point you just have to assume they're not going to screw you over.