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This is an unsubstantiated claim.

Certainly, there have been a fair amount, but not for lack of effort on the OS developers' parts.

It can always be better, and cutting edge, modern OS designs are superior, but it's not like the foundations of NT/Darwin are completely rotten.

Windows NT has been multi-user from the start. In fact, due to microsoft's heavy business focus, it's arguable that it even has better multiple account support (a concept separate from multiuser)