"which has been huge" is the key. Businesses typically grow, mature then decline. Microsoft's revenues had stopped growing. The very basis (PC) of their core franchines (Windows and Office) is in decline. It looks like a business that is past its prime.

Can Microsoft reverse this? It's hard for them, because they are so successful. Most other successes look small: e.g. their incredible and profitable xbox business isn't in the same league. To rejuvenate themselves would be like Google coming up with a better business than adwords, or xerox a better business than copiers. It's like Apple TV: a wild success for anyone else, but for a long time, a "hobby" for Apple.

BTW: Apple's rejuvenation, and creating several new categories, is incredibly unusual.

Can Microsoft maintain constant revenues? Windows still has a huge amount of software for it, and businesses rely on it. They also rely on Office. Phones and tablets have not really began to displace these business roles. Microsoft has made Office for them anyway; and it also has Windows for tablet (on Intel). These won't displace current phones/tablets - but they will enable businesses to keep using their software and Office.

Of course, there is an opportunity for new ways to serve businesses, using mobile devices, that will make the old Windows software obsolete. This is how Windows could die. Office might be harder to kill.