Proportion of HN users whose TVs don't connect to the internet? Significant

Proportion of non-HN users whose TVs don't connect to the internet? Negligible

Even if it did support it, nobody connects their TV to their local network via HDMI, they connect it to their PC. And, as far as I'm aware, no PCs automatically start sharing networking with whichever networks they are connected to.

That could of course change. Could be even a nice feature for connecting your TV to the network via your multi media center, if the bandwidth is good.

Their solution to that is to either use the cell phone network or services like amazon sidewalk where they connect in through your (or a neighbour's) internet-connected device.

Not at all, because it uses the same pins as ARC, so the two features can't operate at the same time, and a lot of people use ARC to get audio from TV apps back to a soundbar or receiver.

HDMI Ethernet is dead, it's never going to happen in consumer televisions because it has a hard conflict with a feature a lot of people actually use.

If people really start not connecting them. TV makers will just start including a cellular modem.

Then you make sure to only use HDMI cables that aren't HEC, in conjunction with not connecting the TV to the internet directly, if you want to keep them isolated.