Yes, you certainly could use MoQ to send multiple streams (WebRTC simulcast style).

There are implementations of traditional receiver-side adaptive bitrate switching with MoQ already today (mostly switching between tracks at group boundaries). There has been interest in exploring sender-side adaptation as well, but it's not clear what that might require and if it's something worth trying to support in the first version of the spec we take all the way to RFC status. Subgroups are something that can be used today, though less experimentation (at least public experimentation) has been done on fully utilizing subgroups at this point. They could be used for independently decodable tracks, though that wastes bandwidth relative to layered codecs. Even layered codecs have some overhead that may not always be worth it. If RTT is low enough and switching is straightforward, it may be that having the original publisher publish multiple tracks into a relay and allowing the end subscriber to switch between them is sufficient.

We could really use more public experimentation with all of these approaches, so if anyone is looking to do that type of research, definitely let us know how we can help support it!