Yep, I understand the rationale for traditional mixers, but that's my point - the M20 isn't a traditional mixer! It creates an entirely new use case - ie soundchecking by recording all channels, and then tweaking the settings. It's only during that specific use case that you'd want solo functionality on the main outputs - not in real-time performance mode.
This is the use case in its simplest form:
I'm a musician in the band, I'm also running sound. The desk is placed next to me on stage.
To soundcheck, I record a quick capture of the whole band.
I then grab an iPad and walk out into the venue to tweak the sound.
First thing I want to do is mute everything except the percussion, and tweak that.
Then I might mute the perc and tweak the two guitars, listening to each in isolation, and then the two together. Then I put the drums back in and adjust accordingly
To achieve this, I need to individually mute all channels, and then selectively unmute and re-mute as I go. If I want to quickly flip between the whole mix and a single channel, I have a lot of buttons to press. Remember, the Mute All buttons aren't available on the app, so I don't have easy access to those.
With a traditional mixer and dedicated sound man, you'd do the above the old way - ie by asking individual musicians to play one at a time. With the M20, we don't have to do that any more!
So, my suggestion would be to have an option to apply solo buttons to the main mix, but ONLY when playing back a recording (Quick Capture or multitrack), it would NOT apply when in normal performance mode, to avoid the scenario you described above with traditional mixers