Who uses a tuner these days?

Anyone who uses an antenna. There’s a bunch of decent channels, like the news, you can get with an OTA antenna.

Modern TV signal is just via Ethernet

No. What you just described is “modern cable TV”. OTA channels are digital signals also.

That would be just a monitor, wouldn’t it?

No. The distinguishing feature between a monitor and a TV is that a TV has a tuner built into it.

There are other things like the variety of inputs and screen position settings on monitors, but those are mostly minor.