That's cool if the Scala bits are mostly static bit, not so much if they see active development. When (say) 5% of your codebase is Scala and the rest is Java,

That's cool if the Scala bits are mostly static bit, not so much if they see active development. When (say) 5% of your codebase is Scala and the rest is Java, and your engineering team prefers working in Java, those 5% become stumbling blocks. The context switching slows you down and increases the risk of introducing errors. Also, a tool chain that will support two languages in parallel, while obviously not impossible, is necessarily more complex than only supporting one.