Each of the languages you mentioned is wildly popular, well run, and not going anywhere. If you chose not to learn Java a decade ago because you "knew better" you aren't any better off than you were before, given that this language is the core foundation many companies and open source projects, maintaining relevance for 20 years. Not learning Swift now means you won't be able to do effective IOS development. Not learning Go means you're missing out on the language that a lot of important systems software is built in such as Kubernetes and Docker and won't be able to contribute to them.

I think also it's inaccurate to call any of these languages proprietary. They are all open source and if you wish you can fork them and make your own. C# in particular is now run by the dotnet foundation which is nonprofit with elected board seats, putting it in a better place for governance than many community driven languages.