especially when the life of a OS seems to be shrinking as the years go by
This has always been the planned business model. The reason XP has been around so long, was the several delays in Longhorn.
One of those major delays being, they stopped dev work on longhorn to work on XP SP2. Then they decided to change the longhorn model at the last miniute (I think due to it was going to break just about every 3rd party software/driver out there, and something to do with exFat).
This caused XP to hang around much much longer than it was supposed to.
With that said, I think the 5 year model is too short myself and should be about 7 years for a major OS change, with service packs adding new functionality, patches and fixes in between. That's my thought, but that is not the way it will be.
So, Vista was long past due for release and 7 is a little too soon (Partially due to the bad rap that Vista received, and that the underlying engine changed and did break some things, not all things, but 3rd party needed a stepping stone to catch up).
Plus, people don't like change until they have to. Win 7 is going to start forcing some real changes. Anyway.
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It was believed some years ago, that Mac would become an OS Only company to compete with Windows.
Mac won't compete fully with Windows till it does.
Again I say, Apple stifles competition much much much much more than MS ever has or ever will.
I will never purchase a Mac because of that. They are far more evil in competition than MS and Bill Gates could ever dream of being.