They're not necessarily lemons at the time of sale though. In general, we don't legislate that products need to upgraded and maintained after they're sold. (Yes, there are lemon laws and warranty requirements for defects--which are at least related.)

However, how would you feel about legislation that required five years of dealer service to be included with every automobile sale? Or other products in a similar vein?

There is an idea of harm to the ecosystem/society with unpatched IoT and other network devices though. So perhaps a heavy-handed approach is justifiable.