1. As a single computer Mac User

. Which is easiest to network, setup a network with roaming profiles, shared resources etc. New PC arrives here, join domain, log on and wait and everything is done thanks to Group Policy. If I need specific software I can use Windows Deployment Services with images across the network.
2. Maybe in the 90s and maybe with companys running older tech. We haven't had to fix an infected computer here in 15 years, no need to restore data and minimal training.
3. There is more Windows software. You cannot dispute this. Linux/OSX is better for free high quality? It depends on your definition of quality. Please tell me some heavily used software that is also available on Linux/OSX and performs better on Linux/OSX (Adobe offerings are the same).
4. Viruses? Not had one, crashes - Hardly ever, we have some buggy software (Nitro PDF) which causes us to restart 1 machine once a week. Updates - yes, is linux never updated?. Upgrade costs - none (check out Microsoft MAPS).
Companies will choose what is the most cost effective for them. This may not be the initial cost, it may not be software licensing, it is the long term productivity, ease of use and interoperability of the business. Employees will then use that system at home as it is familier and allows them to transfer files easily etc. etc.
Rose tinted, limited view goggles anyone?