Most enterprises depend critically on being able to view and edit Word and Excel documents. If there was some urgent need to get rid of Microsoft, they could get by with non-Microsoft applications that support the same file formats. It's easier and cheaper to stick with Microsoft because of the huge ecosystem Microsoft built. That's no accident, it was good business, aided by some questionable business practices which were noticed by antitrust regulators at the time. But it doesn't give Microsoft unlimited power – if they hike licensing fees, the available alternatives will start to look more appealing, despite the (currently) higher cost of finding staff to support them.