Look into the crystal ball
MS started out by targeting the home pc market. Over time they developed the most user-friendly, functionality rich software. They also marketed it very well. To run this software you need windows, so MS made your OS choice academic.
Look back to when MS moved into this market in the late 80's early 90s. If you look at what happened to Lotus 123 v Excel at the start of 90s, everyone ridiculed Excel. Now people barely remember 123. Same thing with Word v WordPerfect, and FoxPro/Paradox v Access.
As MS decides to introduce a new product it starts out with a product that is typically average, and then by version 5 or 6 of a product it generally offers a product on par with the market leaders, and marketed better than the rest ;).
In terms of scalable enterprise level products focused on the Internet, MS only started introducing new products in the late 90s. This is when they introduced Win NT, SQL server and web based technologies such as IIS/ASP.
With NT version 1 was 3.51, version 2 was NT 4, version 3 in 2000..... Extrapolating 5 years into the future where does this leave Unix?
Same for Sql Server when first introduced it was the laughing stock of the enterprise level RDBM market. Now in its 3 major release, MS now has sizeable market share, which shows no signs of slowing at all.
Apply the same comparison to MS Web based technologies a la VS Net.
There is no doubt in my mind that Unix/Java/Sybase/JSP are the current market leaders for enterprise level Web solutions. Give MS another five years, and 2 more major releases for its enterprise level OS/RDBMS/Web Technologies, it is then conceivable that UNIX might go the way of Lotus 1-2-3.
What is MS's major threat? It was *cough* Netscape *cough*, nice tactics Bill! Now it is probably Linux.
For Linux to take market share from windows, it needs to be able to offer software on par with that currently offered by MS. Then it needs to match MS's marketing expertise. I just don't see this as a major threat for MS at the moment, although perhaps in the future if the people behind Linux get a little more organised...