Corporate: the only reason for this is because web apps and sites are written for a particular browser, and not based on standards.

Awareness: when sites don't work, they'll be aware of the problems. The truth is, most users at getting more educated about browsers all the time. It can't e helped that Microsoft has tied browser innovation and standardisation to their operating system.

Familiarity: really fairly bogus as an argument. There is really not that much different between the interfaces of Opera, Chrome, Internet Explorer and Firefox these days!

Those are bad reasons to hold the web back. They don't adapt because they're lazy or don't know better. Making it hard to use obsolete browsers is doing them a favor. They'll like the present when they try it.