[dragonwriter]

> It makes the library unusable for anyone, but open source software.

Using the GPLv3 is more restrictive than that -- it makes it unusable except for GPLv3 software. Even a preference for open source -- even copyleft -- software doesn't mean a preference for the use-based discrimination in the GPLv3's so-called anti-Tivoization provisions.

> If you want your library to popular don't use GPL.

Lots of GPL libraries are popular, though perhaps in some cases less so than they would be if they were not GPL.

> What happens is that someone else comes a long and creates a MIT version. Eventually it gets replaced with the MIT version.

IF you do permissive (MIT/BSD/etc) and a sizable fraction of the people interested in your library are also GPL partisans, the reverse happens (and potentially much more quickly) -- some relicenses your library as GPL, and the active community ends up there. So, making decisions on that basis can go either way.

Use GPLv3 if you want to control what downstream users do and you prefer the particular controls in the GPLv3. Use a different copyleft license if you want to control downstream users and don't prefer the controls in the GPLv3. Use a permissive license if you don't feel the need to control downstream users.

[batbomb]

I don't think can't just go about relicensing a project willy-nilly, even if it is BSD, without the approval of the copyright holders or a contributor license agreement which the copyright holders agreed to which expressly grants the project the ability to do so.

[dragonwriter]

> I don't think can't just go about relicensing a project willy-nilly, even if it is BSD

The whole distinction between permissive and copyleft license is that the former allows downstream relicensing on substantially different terms with very little restriction (typically, something like requiring retaining the original copyright and disclaimers.)

It probably legally requires enough new creative work for the downstream version to be an original derivative work and not a mere copy that wouldn't be eligible for a distinct copyright, but that's still a lot less than a copyleft -> permissive replacement, which requires reimplementation rather than derivation.