The question wasn't whether Rust is good or not, but rather whether it's entrenched enough to not be considered a mere fad anymore. Regardless of your opinion on the merits of Microsoft products or their development process, the point is that it's one of the largest software development companies, and - as enterprises tend to be - is more conservative in terms of technological stacks used for production code.

And? Companies use immature technology all the time, for better or for worse. Why does a few microsoft employees using Rust have any effect on its maturity?