If I remember correctly Unity was a fork of Gnome with lots of patches applied to the point of having both Gnome and Unity on the same system taking effort and being risky w.r.t. stability.

My guess is that they've invested entire Unity vNext on Mir, and if they're dropping Mir, they'll might as well drop Unity 8. That leaves them with Unity 7, and to unfuck a decade worth of changes they've done to make it work with mainline Gnome source.

Basically lots of work to have a 6 year old product back to square one, before they can, once again, attempt to further develop it.

I'm guessing they've decided that's not worth the effort and that their time is better spent helping improve something already mature (Gnome 3).

Touché, but they could have listened to the community back when they announced Mir and Unity 8 too ;)