When was the last time someone forked a programming language? (note: not a runtime, a language)
In practice the only kinds of languages you don't get locked in to are those with excellent interop with other languages. It's got little to do with whether the reference implementation is open source. Kotlin's lack of lock-in doesn't come from the fact that it's open source - that's nice but in practice most community contributions are small. It comes from the fact that you can recode individual classes in Java if you so wish. And in fact, even convert Java to Kotlin automatically, if you're going the other way.
Also, lots of developers have written code in proprietary languages with a single vendor. It's HN groupthink to believe that nobody 'sane' does this. Look at ABAP, Apex or MatLab. All very widely used, even though you may not have heard of them. In the past Delphi was also very popular, and it's only recently that the C# compiler went open source.
> Before that, C# could be considered as a fork of Java, and C++ as a fork of C.
C# and Java might be eerie similar, but by no definition of "forking" might C# be considered a fork of Java. Maybe you could consider it a fork of C as its original name was "C-like Object Oriented Language" (COOL).
Hence, all successful programming languages are open and free.
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