Question: Does a smart teevee need to be hooked up to the internet? I have a dumb teevee so I genuinely don't know. If not, can't one simply not ever give it access to a WiFi network, plug in a Chromecast/Firestick/Xbox, and call it good?
That said, I’ve got many friends who do exactly as you have said and never connect it. IIRC, there is some talk about including 5G modems in near term models to further encourage connection. The data is valuable enough to merit the inclusion.
I've used wired connections or temporary Wifi network in order to prevent the TV from connecting to the internet without my knowledge.
I was able to update the firmware on my LG TV by downloading it and loading it onto a USB drive. Some firmware updates may improve picture quality or compatibility with external devices.
One annoyance is that there may be buttons on the remote that launch built in internet connected apps. It's important to have a TV that can easily switch between inputs.