Be careful about extrapolating from one hire to Microsoft's big picture strategy. MS is a huge company and hiring one person is a nano fraction of their operating cost. If there was a 0.00001 probability that Ubuntu could sweep away their market it would still make sense for them to hire that person. Personally I think the odds are somewhat higher.

Changing over from any system to any other system, even a better one, is a big pain. The odds are always stacked in favor of the known problems rather than the new problems. In making the judgement of how much "system admin time" he is willing to put in, he needs to account for all the time he spends purging viruses, worms, and malware from his windows system.