Nobody had to come after Rockefeller at Standard Oil, but at MS there was Ballmer after Gates.
That and also Microsoft was priced for perfection, both financially and socially. Once you ship Windows 95 there not much you can do, there will never be a leap as big and so a natural slowdown in the rate of innovation feels like going backwards
But Ballmer and a "Windows everywhere" mentality held them back, with several massive failures and right offs on the orders of billions of dollars.
There was nothing natural about Microsoft's slow down in the OS market while the computing industry was going through profound form factor and OS changes, and rocketing sales, for a decade or so.
There was. The DOJ is the reason. A huge part of the setback was the morale hit inside the company.
Kinda like "what's the purpose of all this work? If we are successful in the way we want to be, they'll break us up"
Microsoft was forced to make a leadership change back in 2000 to avoid being broken up.
Gates had to step down because he was the reason why the DOJ went after Microsoft.
Nadella wouldn't be at the helm if it wasn't for the first forced change from Gates to Ballmer