I'd buy an ATV that had a OTA tuner (ATSC3.0?) and coax antenna connection. That will almost for sure never happen. But...it really would be cool.
Would a single tuner be enough for everyone? Probably not. So build in two? Is two tuners enough for everyone? Probably not. So build in three? DVR service (which benefits from more tuners so one can watch one thing live while another thing(s) are recording)?
You can have what you want immediately with
Silicon Dust HDHomeRun boxes and apps like the terrific
Channels DVR app that work with them. I have this setup myself and love it. I have 4 ATSC 3 tuners + 4 ATSC 1 tuners + 3 Cable tuners in 3 HDHR boxes. Channels app brings it all together. Synology NAS (one of many options) acts as an any-size, whole home DVR. It all "just works" and Channels is the most-used app on AppleTVs in my household because of it.
Along with making every AppleTV in the home a combined over-the-air + cable television box, it also makes every
computing screen in the house have access to the same live or recorded television too. I could add any number of additional tuners if my household needed more.
To me, this is far superior to Apple deciding on the number of tuners to build in (which will almost certainly always be too few for most people) and limiting what you can watch through those tuners to LIVE tv only and only on the television screen attached to that AppleTV.
If you actually want the core benefits of your wish NOW, HDHR box(es) + Channels or similar apps is THE way that "just works"
right now. Look into it and you can have any number of ATSC3 tuners for your AppleTV in only a few days if you want it... just outside of the actual AppleTV box.