&gt; It makes the library unusable for anyone, but open source software.<p>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&#x27;t mean a preference for the use-based discrimination in the GPLv3&#x27;s so-called anti-Tivoization provisions.<p>&gt; If you want your library to popular don&#x27;t use GPL.<p>Lots of GPL libraries are popular, though perhaps in some cases less so than they would be if they were not GPL.<p>&gt; What happens is that someone else comes a long and creates a MIT version. Eventually it gets replaced with the MIT version.<p>IF you do permissive (MIT&#x2F;BSD&#x2F;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.<p>Use GPLv3 if you want to control what downstream users do <i>and</i> you prefer the particular controls in the GPLv3. Use a different copyleft license if you want to control downstream users and <i>don&#x27;t</i> prefer the controls in the GPLv3. Use a permissive license if you <i>don&#x27;t</i> feel the need to control downstream users.