Call it educated speculation based on years of experience with similar decisions.
How big is the team? How many years of maintenance have you had on those projects? I remember C++ looked pretty awesome for a while. It was certainly great for one-man projects or small team. Then the problems started piling up. Thing is, C++ didn't have any good alternatives (maybe now we'll have Rust); Scala does.
Scala is not C++. Scala has much better features to keep complex pieces of code separated from the simple pieces. Also C++ design philosophy was totally different from that of Scala's - C++ values performance over everything else, while Scala's top priorities are correctness and type-safety.
Nope. C++ is almost a superset of C (there are a few minor differences, but most C programs are correct C++ programs).
Even the most basic Scala program is not a valid Java program. They are just two different languages targetting the same platform.
If making analogies, a much closer analogy would be to say Scala is to Java what D or Rust is to C++.
As of lots of baggage - can you elaborate? Only null comes to my mind and it is never an issue - it was left because Scala is practical. Sure, there are some limitations imposed by the target platform like lack of TCO, lack of efficient suport for tuples/value-types, or RTTI for generics being not powerful enough, but Java-the-language has nothing to do with it. The same limitations apply to Kotlin and Ceylon.
How big is the team? How many years of maintenance have you had on those projects? I remember C++ looked pretty awesome for a while. It was certainly great for one-man projects or small team. Then the problems started piling up. Thing is, C++ didn't have any good alternatives (maybe now we'll have Rust); Scala does.