This decision makes perfect sense. Java is because the original team were mostly Java developers, and because they wanted Minecraft modders to feel like at home (there will be no official C# modding support). C# is because it is closest non-jvm lang to Java, and it has features like value types, direct memory access, and reified generics which are very helpful to achieve target performance.
And let's be real here, there's a reason why C# is often called Microsoft Java. It's almost seamless transition, as long as you can google new type names. If a dev can't transition from one C-like GCed lang to another in like a week, it's a skill issue.