"So it is not at all clear that Linux can close that gap"
Time is on Linux side. It has become obvious that there are only a finite amount of things left to do to the desktop that make any difference to the end user. After that, it is only a matter of time for Linux and FreeBSD to catch up. Longhorn is the last hurrah for Microsoft, mostly useless bloatware.
"Everything that can be invented has already been invented." (1899 U.S. Patent Office official, though the attribution is possibly apocryphal)