Not sure if you have spent much time inside an enterprise. It doesn't matter if there is no excuse, they won't do it, and calling them mythical isn't going to help convince them.
The reason I say mythical is that I have never heard these arguments from someone who actually works for one of these enterprises.
(This is not surprising to me, because I know that if I worked for a company that only allowed IE6, the last thing I would want would be for developers to perpetuate my misery by continuing to support it.)
If someone can actually tell me the name of a major corporation that only allows IE6 to be installed on its workstations, I'd be genuinely interested in hearing it.
I worked there for 4 years. As a developer, I was able to install other browsers (that was against the company IT policy, but I didn't care). When I was working on the business side, though, I didn't have admin access to my machine, and so I had to use IE6, like everyone else.
All the internal apps were built to work on IE6, too. And there are a lot of internal apps. Half of them wouldn't display properly on Firefox when I tried opening them there (this was a few years ago, I didn't try Chrome or Safari).
