It is almost certainly sufficient for all extant models of TVs. It might not continue to be sufficient in the future, but for now, it probably is. And as another commenter pointed out, if you are really that worried about it, TVs are still among the easier electronics to open up, so you can just physically remove/destroy the networking hardware.
If a manufacturer ever _does_ ship a TV that will look for and auto connect to open networks, I'm curious if they would think to still do that, even if the TV is connected to a closed network. My guess is that they probably wouldn't (generally not worth the time to program for edge cases). So you could connected to a virtual network that doesn't have WAN access. The TV is connected to a network so (likely) wouldn't look for open networks, and still can't phone home or download updates.
2. It is possible that it's currently being done, just not discovered yet. Consider the Volkswagen emissions scandal, Amazon ring spying scandal, Tesla spying scandal, etc. They aren't known about until they are discovered and reported to the correct place. That's the nature of people who flout privacy. It's probably happening somewhere already and just hasn't been found.
I assume that anything that has the ability to leak my private information, probably is leaking it to someone, and then take whatever steps I can to mitigate it. I'm not claiming absolute success, but at least I'm trying!
Furthermore, the more technology that you know about simply makes you more aware of the possibilities. I worked for a cell phone company a decade ago who also sold services to electronics manufacturers to add SMS-based communication to otherwise unconnected devices. In other words, if wifi is down or blocked, an SMS could be sent (small payloads, of course) as a backup. In grad school I listened to a presentation by a small, local newspaper that bragged about tracking people around town via bluetooth and wifi that people left enabled on their phones, and that their sensors were hidden in the newspaper stands around town.
So, yeah, I don't trust anyone who has a financial incentive to spy on me.
There is a reason I preceded it with "almost". And I completely agree with everything you said. I just think that "at least I'm trying!" covers "not connecting your device to your home network".
If a manufacturer ever _does_ ship a TV that will look for and auto connect to open networks, I'm curious if they would think to still do that, even if the TV is connected to a closed network. My guess is that they probably wouldn't (generally not worth the time to program for edge cases). So you could connected to a virtual network that doesn't have WAN access. The TV is connected to a network so (likely) wouldn't look for open networks, and still can't phone home or download updates.