With regards to the future of Scala, the problem is that Scala isn't even the incumbent of being "the next Java". Java (8) seems to be the next Java. Scala is already quite old to be so far behind, and there are several alternatives to Scala that don't suffer from Scala's shortcomings. My take on Scala is that it's a very interesting language that has taught us a lot; time and time again, when faced with the choice, it has taken the research-oriented route rather than the developer-oriented route, to its detriment; while gaining strides, it has failed to capture the hearts and minds of enough developers; there are languages that have learned from Scala's mistakes, are more developer oriented, and are more suited for widespread adoption.

As to the few constraints, keep in mind that my point is that those constraints are or at least directly lead to benefits. So few constraints mean few of the benefits. Scala mostly enjoys the "few lines of code" benefit that Ruby developers like, but not the more important benefits of a solid, well-considered, easy-to-maintain language.