Maybe i can help against that warmth ;)

Could some rogue javascript establish a connection from your browser to your printer?

Oh don't :)

I'm actually worried that some newer smart devices might be set up to use well known public wifi services that are available from consumer routers.

Just sitting here I have public "EE Wifi X" and "BT Internet" which it could connect to if configured at the factory to do that.

Then I am boned.

Check out Amazon Sidewalk—if any of your neighbors connect a device supporting it to the internet, your devices will find them and use their internet connection if you don’t provide one! Apparently their network covers like 90% of the US population (at a low data rate), and they’ve made it possible for non-Amazon devices to implement it.

Ok so my next house: must have Faraday cage.

Maybe, but it has been a lot harder than doing the trivial thing for a few years already.

If it can, it's a vulnerability that has to be fixed.

Rouge as in red, or as in rogue?

You need to do some rouge teaming to explore this vulnerability.

Fixed the typo. Even though its not as far fetched, i am not talking about cosmetic JS :)