Cellular modems seem unlikely, because both they and the service plan they'd need are too expensive to be justified by any benefit to the manufacturer.

Not so far fetched when your advertising revenue is in the tens of millions of dollars.

They just need this revenue by tv to be higher than the price of the mobile plan per tv. For a few Gb per month it may be relevant in the not so distant future.

Yes. Considering 90%+ of users will just connect their TV to their WiFi, shipping a cell modem with every TV to catch that last 10% seems like bad ROI.

I could however see this happening in the future in response to regulation. For example some "think of the children" rule that makes TV makers responsible for blocking mature content.