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I worked there, you didn't, so please leave your biases elsewhere.
If you have ever actually shared responsibility for an IT department of that scale, feel free to share your own experience. Until then, please don't try to cover up an obvious avoidance of clear and rational thought with cultish slogans ("mission critical") and fanboy enthusiasms.
Managers care about reliability, but they care more about money. There's no reason to spend money to achieve a level of reliability and performance that are not needed. If a business could operate successfully using a squadron of Sinclair QL machines, that's exactly what they ought to do.
None of the servers in question -- NT or those running Solaris -- ran IIS. (There are other kinds of servers, you know, besides web servers.) We ran only a few web servers -- Apache on the Sun's and then Lotus servers when we ditched the Sun boxes for Wintel.
We found Sun support to be costly and inadequate. We found Sun hardware to be no more reliable than our Wintel boxes, and considerably more expensive to maintain. (Commodity hardware and peripherals were, by and large, unavailable for the Suns. If something broke, you paid Sun prices for Sun hardware. When your boss knows that a Sun hard drive costs 10 times as much as an equivalent Wintel drive, you won't bew able to convince her that the Sun drive is worth cost.
Others may have had different experiences. In my case, the cost of Sun and Solaris weren't commensurate with performance and reliability.