If you think of it in terms of the words "need" and "alternative" then you will find a reason not to use Scala. Personally, I have found it painful to watch Java tack on Scala features years late and in awkward and clumsy ways. Encountering ideas first in Scala and then in Java is too often like learning math from a modern textbook and then looking at the same math in a textbook from a hundred years ago. You appreciate how decades of small improvements have made the subject less intellectually taxing despite the fundamental content not changing at all. For Java to end up on the wrong side of this comparison even when it implements features years later, to me marks it as a language in terminal decline, no matter if its popularity has begun to reflect it yet.