Why should it be just the IoT devices that get the insecure network? Why not just stop trusting the LAN altogether and instead use technologies like HTTPS and DoH to ensure privacy on the important devices? That seems to be the way the tide is turning anyway.
Personally I'm all for that but people & packages seem to be pretty promiscuous about listen address defaults and assuming everything behind a routers NAT is trusted.
Treating the network as untrusted is good but as long as some people are paying for service, traffic and bandwidth there are reasons to not allow anything to use your network. And there is also a legal question of liability if someone is not quite above board from your IP.
