> For my current project I have a very low level spec/architecture, that says class Abc does Xyz, classes A1, A2, A3 implement a strategy pattern, etc. It's designed with C#, VB.NET or Java in mind, but the dev team would prefer F# or Scala.

This sounds like a horrible environment to work in. If you don't trust your developers to make those kind of decisions for themselves then what are you even paying them for? That gives them no ability to exercise their skills and I would not expect any good developer to stay long in such a place.

Frankly it suggests a level of brokenness in your process that I've always found unfixable. I'd advise finding a better job.