Microsoft probably has tons of internal libraries that are modified and shared between many different teams spanning multiple orgs.

I think a pivot like this is an incredibly complicated thing for a company like Microsoft to perform. It requires multiple years of planning from the ground up to port internal libraries and tooling, train engineers on Rust best practices.

Not to mention the tricky business of going around and hiring Rust developers at the scale MS needs. :)

I think, simply waking up one day and changing the language / frameworks / tools used by your company is a privilege only smaller companies and startups might enjoy.