Can you point to at least one "established set of enterprise "web" applications" that supports this claim?
Aside from corporate users, individual consumers who aren't necessarily computer-savvy (like my parents) don't have the knowledge (or desire) to download additional software and install extensions, plug-ins, etc. They go with the default browser of their O/S.
No one wants to rewrite their intranet just to support a marginally better browser.
So there 350,000 users or so..
(go there in any browser except IE). This is pretty much the case for all PwC webapps. PwC = >25,000 U.S. employees.