I've yet to see a linux distribution supported for even 7 years, let alone the 10 minimum guaranteed by MS. Sure, you can in-place upgrade linux to a new version of the distro, but Windows allows in-place upgrades now, too. You have to pick your poison here. If you are updating, you're gonna have some of the same stability and migration issues on linux that you'll have going to a new version of Windows. If you're not updating, you're eventually running into the same security issues you get running old Windows. As far as *real* long-term stability goes, a linux server might run for a few years without a reboot, but IIS clusters well enough, and Windows can guarantee you a decade of security updates for a platform. I have to get it the edge here.
Additionally, if you're hosting yourself, and you run VMs, once you've licensed data center edition on the basic hardware, you can spin up as many Windows VMs on that hardware as you need at no extra cost. Really. The basic data center license doesn't cost as much as you seem to think it does. My last purchase was about $200. That's a rounding error even for a startup. I'm in the Ed market, so I get a pretty good discount, but this isn't that far away from the typical. Big customers get extreme volume discounts, small startups can take advantage of programs like BizSpark, and there's a reasonable plan for most of the rest in the middle.