I don't think that's the right argument. Re-writing few parts or writing entire app in new lang, which one would be better for app developer?

If only you could write a new service in a different language and then make requests to it... oh wait, you can!

At my last job we had a Node.js app talk to a Scala backend and the other end of the system ran PHP. I mean we already had like 30 servers at least, so dedicating one to Node and a few to Scala wasn't a problem.