Internet Explorer 10 Drops Vista Support

It's no different than Safari 5 supporting XP (2001) but no longer supporting Mac OS 10.4 (2007).

My first guess would be that both companies stack incentives via artificial barriers like that.

In the real world, I suspect/fear businesses just sit on legacy systems until something critical breaks.

I imagine it's easier to get a business to upgrade to a new browser version than it is to get them to migrate to a new browser entirely. (Blame changes in shortcuts, UI differences, integration support, etc.)

That said, these policies hold back the web. Cute marketing tricks[0] don't fix it. Sure, it says to upgrade. The newest version is unavailable for your OS: now what?

[0] http://ie6countdown.com/

Is it because to justify shipping the browser pre-installed (to regulators), they have to 'tightly integrate' the web browser runtime available in the OS with the browser itself?

Well, some companies make money selling browsers while others make them selling operating systems. My 2 year old iPod touch is no longer supported for instance so I'm stuck with an older safari. Same thing for my laptop if i chose to use safari.