If they would switch to more efficient languages, they could probably run on half of their current servers.

But there's the huge cost of rewriting something you already have. Will that affect your total cost significantly, if you can run on less servers.

I.e. it's too late to switch now. There needs to be a significant business reason to switch. "Stuff-that-kinda-works but needs more hardware thrown at it" is probably not a good reason.