> You don't really have to test, it is possible to make commercial or IT > applications in-house. > > I still have the impression of a discussion in a vacuum.
I don't necessarily agree with the "killer app" theory. C# is immensely popular but doesn't have a particular killer app. However, in the case of Rust, it was really a language created for a particular purpose which interesting because you can really observe the goals and its purpose in the language (always check utf8 for example, important for parsing web content in Firefox). However, most languages are not created for a particular purpose. What I have observed, what sets the winners apart is a proper IDE with debugging capabilities and all the luxuries you can think of. Qt isn't a language but QtCreator is a big factor that it is so popular. Then we have syntax, aesthetics and ease of use plays a big role. People want to get from A to B as quickly and easily as possible, the languages that provide this are winners. Then we have libraries, languages that have a big amount of high quality libraries are winners. With leading languages as C# Java Python You can observe that this is true for all of them. Major company behind the language is only valid if that player (like Google or Mozilla) invests heavily in the language eco-system like creating libraries. Major player only is not enough, they also have to create additional value for the language.
