I don't think it's that simple. When starting a project, of the available options, which one gives you the most leverage toward building what you're trying to build? What is the cost of using that (training people plus machine resources)? Pick the best cost/benefit tradeoff and start to work.

For an existing project, the bias is to not change unless the new language gives you a massive improvement. Re-writing is always harder than you expect, because the existing code base has more bug fixes and handled corner cases than you think it does, and your re-write has to preserve all of that (or suffer the consequences of failing to do so).