I don't think that it will be a big issue since the major reason for end users to still have IE6 around is inertia.

The major reason for end users to still have IE6 around is that they use proprietary applications which were written with IE6 in mind, and they see no good reason to spend money to upgrade these applications (if the vendor is even still interested in doing so) if it still works.

So, as long as there is a way to opt out, I imagine there are a lot of corporate folks who will do so.

I think these are best addressed with the X-UA-Compatible header, and various other ways to trigger the use of an older IE engine. Otherwise, users are offered a choice between broken legacy internal apps, or breakage on most of the rest of the web.

If that requires a change to the legacy application then it's never going to work.

There's a reason companies can't upgrade from IE6 without a significant cost, and it is usually one of the following:

1) The company has lost the sourcecode, and the original engineers have left.

2) The company doesn't even have the source code, and has no rights to modify the application.

3) The application is vendor-supplied, and said vendor requires a large cash payment to make the 'custom' change.

4) The application is vendor-supplied and the vendor refuses to support IE6 in the version of their application that the company have deployed. Their new, super expensive version does support more browsers.

5) The company has little control over their IT. Their business depends on a thousand tiny applications built by countless different parties of varying ability. Simply auditing the whole mess is too costly.

So lots of companies take the easy (and cheap) route out and decide to stick with an ancient browser.

Adding a single header can be done as simply as using nginx as a 'transparent' proxy in front of the app. No need to make application changes.

or can be added to policy/registry that open http://legacyapp:6583 in ie6 frame.

Modern versions of IE do not contain a compatibility mode for IE6, hence the problem. (The official solution is virtualization.)