Hi Jakob & all!
Thanks for bringing this up.
My preference would have been to keep MIT as it was in the beginning. But then, as now, I support whatever the project leadership thinks is best. This was the thread:
I was part of a team that moved Bootstrap from Apache to MIT for easier inclusion in (L)GPL web apps. Over 350 committers had to agree and in the end, we couldn’t reach some people, and some commits had to be rewritten: Migrate to MIT License · Issue #2054 · twbs/bootstrap · GitHub → Not fun. Syncthing is still young. The longer we go, the harder it will get to make such changes. If we are to make a change (and I prefer that we do), let’s do it now, even if that means rewriting some commits. It’s much easier to move in one direction (towards GPLv3) than the other (MIT) because you can say the project is now license X, and it includes all this previous code of a more permissive license. This being said, we see that licenses are a touchy subject and some get an uneasy feeling when a project changes it. So ideally, we would stick with this license for foreseeable future.
The most important thing is that Syncthing can be deployed to all kinds of platforms and it becomes a universal solution to sync anything with anything. So an official iOS app will be a big win. Down the road, I would love to be able to buy a phone / tablet / TV / hard drive / NAS / router / whatever with Syncthing pre-installed. I am not worried about some trying to close up the code. Code evolves and especially for a P2P app, it’s in everyone’s interest to upstream and keep in sync with the community.
The license shouldn’t make it unduly difficult to include code we may want to include. I am not aware if this is or not a real issue, but just mentioning it in case.
Aral Balkan reported being OK with MPLv2 or MIT:
So this reduces friction to collaboration. I realize projects have different goals, and code bases will diverge more & more over time, but even if we only get a few fixes, it’s a good thing. And we can hope for more.
Ideally, all downstream projects (installers, wizards, system trays, etc.) would use the same license to minimize any potential friction of collaboration or transfer. I expect they will all follow suit, if asked.
So all in all, slight preference for MIT, all fine with MPLv2. Let’s get this done and get back to making Syncthing even more awesome!!
Best regards,
M 
Some related links: The Free-Libre / Open Source Software (FLOSS) License Slide (MPL 1.1 not 2 on the current chart) Discussion of license implications (GPL, LGPL, MPL2, MIT, iOS, App Stores)