Comment by nine_k

There are things that are compact but build an interoperability ecosystem around them. Various compression algorithms, cryptography algorithms, communication protocols benefit from having a permissively-licensed implementation. Producing a closed-source fork won't make much sense, and where it does, won't damage the ecosystem. If I invented a new image compression format, I would like to see it supported everywhere, including all possible closed-source software.

There are things that are complex enough, and build an ecosystem on top of them. Producing a closed fork may split the ecosystem, and strangle the open branch if it. These things should use a copyleft license, or maybe dual strict copyleft + commercial license. Linux, Python, Postgres, Grafana, Nexcloud are good examples.

WordPress did it almost right, it uses GPL v2. But to force contributions from hosters, they should have used AGPL, which did not even exist at the time.