Correct me if I am wrong, but to my understanding, both Unity and Mir are open source, and nothing stops other distros from adopting it.
Cf. the effort necessary to get Unity running on Arch: https://github.com/chenxiaolong/Unity-for-Arch
You have to rip out and replace a lot of software, all the way down to GTK itself, just to make it work.
Unity, on the other hand, is a bloody desktop environment. You can install dozens of those (or could, if there were that many) in parallel without any modifications… with the sole exception of Unity.
But then again, you don't think that there are dozens of desktop environments. Arch supports nearly two dozen by itself (https://wiki.archlinux.org/index.php/Desktop_environment), and that list doesn't include things like fvwm, or tiling desktops like xmonad or i3.
Edit: To clarify, the point being made is "if you have to modify a bunch of stuff, then the software is not free". Systemd requires a lot of modifications - it's not a 'drop in' replacement. Using the same argument, systemd is 'not free'. It doesn't matter how 'invasive' the component is, the argument is about modification effort.
I think the argument is wrong - Canonical patches a lot of things for Unity, but those patches are publicly available and are not done to obfuscate the code. Basically the argument here is that no-one should be allowed to publish free software if it can't be simply dropped into $RANDOM_YAHOO's system.
From my own experience, migrating machines from sysvinit to systemd is trivial. Systemd does not need all those optional bits and is happy to work with syslog, ntpd, cron, etc. pp., can pass through sysvinit init files, and so on. If you actually use it instead of relying on fudy reports, it's a breeze to work with.
Recompiling half my desktop, and then recompiling all userspace programs using any of the involved libraries, is much more invasive compared to that.
Another approach is looking at what you have to do when building a system from scratch: Here, systemd's "modification needs" are obviously close to zero (because the system is built around it, you can use journald/timesyncd/etc. from start, or set up rsyslog/ntpd interoperability for systemd). Userland software might, at most, need unit files to use them. Desktop environments et. al. work without dedicated logind support or anything. Unity, however, still requires me to scrape together patches out of Canonical forks of various programs and libraries to get it to work. It's still massively invasive and requires far, far too much effort.
Other distros could try to port Ubuntu projects, but why would Canonical accept their patches upstream?
Though I dislike Unity and Mir, I think RedHats understanding of "Open" Source is a lot more disturbing.. https://lwn.net/Articles/432012
Not that I disagree with your comments on Red Hat, but let's get our facts right.
The person you responded to is correct though; a large factor in starting systemd was the CLA of Upstart. Above even highlights this; you cannot really say that RHEL6 used upstart in any way.
As a result, rather than working towards a united goal of empowering users with the power of FOSS tools, we started having these ego battles of "mine vs your distro", "gnome vs unity", "systemd vs upstart", etc.
Its true that Linux has choice and FOSS is all about having choice. But that has gained quite a negative reputation now. Earlier, choice meant: "If you need more features, use X package instead of Y".
Now it means: "Since I have got choice, I'll keep switching my distro every couple years, and since I'm a unity fanatic, why should I keep it compatible with other distros? Everyone should use unity and ubuntu."
Unity and Mir are no doubt open-source, but if you build a automobile engine that fits ONLY to your car and call that engine open-source, what use is it!