"Open source" is more than just GPL. MIT, for example, can be made proprietary. And yes, the last version of that MIT-licensed source will exist forever, but,

"Open source" is more than just GPL. MIT, for example, can be made proprietary. And yes, the last version of that MIT-licensed source will exist forever, but, in practice, forking and maintaining that fork can be a tireless, difficult, painful endeavor that most people will not have the time and energy for.