KWin is GPLv2+, this means that a user can choose between:
* any other later version of GPL if they exist
Now if we link to Wayland, nothing changes because Wayland is MIT licensed. If we link against the Mir server libraries the only possible choice is GPLv3. This means we take away the freedom to choose the license.
I know that there are people saying that this sounds like a made up argument and that GPLv3 is the best thing in the world and everyone should switch to it.
But I see this more pragmatic. Ten years ago everybody would have agreed that GPLv2 is the most awesome thing in the world and that there is absolutely no need for a GPLv3. Nowadays we have a GPLv3 which is even incompatible with GPLv2. What if in future there is a GPLv4 and we would want to use it? We would not be able to do so because we have a dependency on a GPLv3-only library.
For me it's also that the developers who started KWin did the deliberate decision to make KWin GPLv2+ instead of GPLv2-only. Also we did the deliberate decision to not change the license to GPLv3+. Now KWin is a rather old software (from 1999) and I think this license choice is part of the inheritance which needs to be preserved. You don't gamble with something which served us well for over a decade. Given the age of KWin I'm more thinking in what will us help over the next ten years and not about how can we get to be used in Ubuntu tomorrow.
So in summary: for me as maintainer a GPLv3-only library is not an acceptable dependency.