Seized the day!
There were many moments in his tenure when he could have changed the game, given his dominant position in PC operating systems. All the way up to the phenomenon of netbooks, their cash cow could have been leveraged to create new markets. Instead he milked the cow until it had no more to give. When that happened, he put a clown suit on the cow and told everyone he was running a circus. A few million people were curious and bought the new product, and the rest of us said -- WTF!?
Ballmer ran the company by the numbers. And tech companies that are run by the numbers end up doing what Microsoft did, they run out of room to maneuver.
Having made a big bet on Windows as a server platform at the end of the 1990s, I wish Ballmer had been willing to bet alongside me, and created a lean platform for apps that was as economic as their main competitor which still to this day is nowhere near as easy to use as Windows. What might have happened then is the computing priesthood might have been knocked on its ass in the same kind of paradigm shift that created Microsoft in the first place. Today, more than ever, running servers is the domain of experts. Microsoft, if it had any imagination, could have changed that.
And had they done that who knows what kind of software might exist today? We have no idea because we didn't go there.