I think Unity was pretty bad when first implemented, so, as far as the community was able to see, something they liked was being forceably replaced by something that didn't work well. I was not able to see into the future that the next release of Ubuntu would have a Unity that worked better.
My fear about the MIR stuff is that it will break many applications that weren't specifically written to run on ubuntu. the Generic Linux apps of the world. But I haven't played with it yet, so I don't really have any idea.
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