A lot of the best people left MS. People forget that MS was so completely dominant that it attracted a lot of talented people who wanted to do big things. Since then the competition from the FANG companies have brain drained them, not helped by MS insistence on paying less than their peers, sure there is no state income tax but the company shouldn’t be trying to capture all of that difference.
The net effect is that not only are they not capable of building things as well as they used to, they can’t even maintain what they already have. And how would anyone go about fixing such a culture. I think that’s a big part of why MS has been so generous to OpenAI while also keeping them at arms length.
Inertia is incredibly powerful and big companies can remain successful for long after their technical competence dissipates.