1/ Mir is dependent on a proprietary Boost software license, and, should it follow Canonical's normal path, will require you to assign your work to them to contribute. Properietary dependencies and a requirement to hand over your code to a commercial entity can be considered open source, in much the same way that, say, OpenSolaris was, with similar pitfalls. For something so foundational, that's not great.

2/ NIH is a bad reason for parallel projects.

3/ The major reason that nothing has unseated X, despite its very longstanding, well-known problems, is because nothing has had momentum until Wayland. Canonical seem determined to deep-six that momentum with a piece of software that will require other Linux environments to buy into a company-owned replacement. This seems likely to fragment the effort it a toxic way, with two incompatible replacements splitting not just graphics development efforts, but splitting app developers.