I think "strictly superior" makes sense in the way that Scala was never designed or intended to be a worse-is-better language like Java and people were willing to make that happen by evolving the language. It was never intended to be a Java-with-more-concise-syntax unlike some other newer languages.
If something was clearly worse, it would have been changed years ago already. That some design is still there might suggest that some people just have a different minority opinion on some aspects. (Assuming that "strictly superior" does not mean "100% of devs agree 100% that this is 100% better than Java".)
Why would you create a new language and then keep a huge part of what developers interact with on a daily basis in such a broken shape as in Java? (Or rack up a massive amount of complexity and break your typesystem by trying to put lipstick on the Java collection pig? (Kotlin).)
Hindley-Milner means that you – unlike Scala – don't need to provide any type annotations at all as they can all be inferred.
This comes at a steep price, as HM-inferable typesystems are very limited, and most languages have in fact a typesystem that might be based on what HM can infer, but have extensions that require type annotations.
In the end Scala's requirement to annotate input parameters is not a big deal, as this only enforces what Haskell/Ocaml/... consider to be best practice anyway.
- It is mature and rock-solid, but still manages to evolve, fix issues and simplify/remove features. Most other languages are purely additive, meaning you will have to carry on all the baggage since the languages' inception.
- It is a language which is interested in identifying the best way to solve common programming issues and spares you with all this ideological "OOP/FP is bad" bullshit.
- It has the largest ecosystem (plus it can use all of Java's, too).
- It has the best tooling.
- It has the largest community.
- It has some great learning resources, like the three (or four?) Coursera courses starting in a few weeks.
- Its JavaScript backend is stable and production-ready, Scala-Native is being worked on.
> That said, I come from a C++ and Python background, so do I need to know Java well before learning Scala?
Java knowledge might reduce the amount of learning (as many Java things like the JVM also apply to Scala), but isn't really necessary. You will be fine.