My prediction is, Ubuntu and Mint will grow side by side, covering a much larger market segment than they could have otherwise. And because Cinnamon is such a darn simple extension, so to speak, it will eclipse, ever so slightly or more, its parent. You must never forget that Windows people will not take lightly to a left-positioned launcher, but they will almost naturally flock to something that looks like Cinnamon. Forget about the geeks, even though they are the reason why it was born in the first place. And it seems to be working, as you can see the Mint popularity soaring. Something is being done right somewhere.
MATE will follow, to a much lesser extent, but to all those who find Cinnamon too bloated, too heavy, incompatible, or simply do not wish to use Unity, Ubuntu or alike, it will be a great supplement for a productive and efficient computing. However, it will NOT make a revolution of its own, because Gnome 2 failed in the same space. So it must rely on Cinnamon for some brotherly love.
And so, the Linux desktop world will evolve to a new level, where only KDE will be the familiar player from years gone. You might think that too much forking causes strife and leads to significant fragmentation, and in most cases, this is true. But you cannot treat Cinnamon or MATE as separate pieces, which is what makes them so useful. Hence, the reason why they will emerge as dominant desktops in the years to come. The future of anything is something that looks like the present.
Highly interesting to read!