The reason Canonical is scrapping Mir and Unity 8 is cutting costs. According to rumors, canonical lost 300 million pounds last year. Neither the phones nor the desktop has been making any profit and the desktop can be continued with Gnome 3, which will be a lot cheaper for the company. And apart from all personal preference, any desktop environment, compositor and display server could be used to make the most usable open source desktop in the world, as Mark put it.
Unity 8 was tightly integrated with Mir, it's whole point was to make convergence easy and beautiful. So if you want to discontinue Mir (in a time were systemd and pulse audio are finally accepted, this is probably the most hated piece of code on gods green earth) and you rip it all out of Unity 8, you end up with a useless brick and a lot of work is needed to make if function again. That would still be costly, so you'd have to scrap both of them. Then what would happen with Ubuntu Touch? It is built both on Unity 8 and Mir, so it would be fairly useless without them and it would loose its main selling point: Convergence.
Convergence has been the one reason why canonical started Mir, Snaps and the transition to Qt in Unity 8. And if you want to get convergence between phone and desktop, you have to push on both ends. So scrapping the phones and keeping Unity 8 and Mir does not make sense, Scrapping Mir and phones and rebasing Unity 8 to Wayland would be costly. So this decision had to be made.
Nevertheless Unity 7 will live on with 16.04 at least until 2021 and might even stay in the official repositories after that. Unity 8, now called Yunit, will be continued in a joint effort by UBports and yunit.io; UBports will also continue to support Ubuntu Touch, so not everything will be lost.
Now let's all take a moment to thank Mark Shuttleworth and the team at Canonical for all the amazing work they have done. We should not forget that they always have and will continue to provide all of us with a great free and open source operating system.