I would guess it will be very similar to the NTSC to ATSC transition. Remember when stations would broadcast in both SD and HD because most people didn’t have an HDTV? It will probably be the same with stations broadcasting both ATSC 1.0 and 3.0. If your TV doesn’t support 3.0, it will just play the 1.0 broadcast. If the station only broadcasts 3.0, you will probably have the option to use an external converter box.
As ATSC 3.0 becomes more prevalent and support becomes standard on new TV models, all consumer audio companies will follow suit and begin supporting Dolby AC-4 including Sonos. But I expect this to still be a few years out.
How would the Arc currently handle Dolby AC-4? I would imagine you would either get stereo PCM audio or no audio at all.
With a Sony X90J with ASTC 3.0 tuner and Sonos ARC, the broadcast recevied by the TV is 1080P and AC-4 (vs. 1080i Dolby Digital/AC3 on a regular channel). On my phone, Sonos App displays this as Dolby Digital 2.0 and at least there is stereo sound.
When AC-4 is supported, it can also support ATMOS, pretty cool to get it from the air.