That still doesn't make any sense. If they want to collaborate to build an advertising profile, your public IP is all you need. Otherwise if they're not collaborating, what's the plan, find 0days in random IOT devices and hack them? I might be concerned about random chinese IOT devices doing that, but not devices from western companies.
Data brokers are buying from multiple sources because maybe a home has a keurig but not an LG tv. Or an LG tv and not a keurig.
The plan for the likes of Keurig is "These data brokers will give us free money for data from our coffee machines? Where can we sign up!". It doesn't even matter if the money translates to $0.01 per unit sold. That's probably the most disgusting part.
Right, but OP's premise is that putting everything on the same LAN is somehow even better for the manufacturers/data brokers/ad networks/whatever, which doesn't make any sense. The only thing that actually matters is a device with internet connection.