1. can't be set to be the default browser;
2. has no address bar, bookmarks menu, or any other "user navigation" feature besides interacting with the pages themselves;
3. and, most importantly, requires a GPO for each set of Intranet domains you want to grant it access to, with a hard-coded start page. All navigation to domains outside the allowed list would trigger the default (modern) browser to open the link instead.
This effectively would make IE6 not a web browser, but just another runtime for ordinary installable software, which just happens to run mostly by loading external code from the web. (Sort of like node-webkit, actually.) Microsoft could re-brand it, with their usual flare, as the "Legacy Intranet Services Application Framework", and tout it as a "transition tool" for any enterprise "lagging behind."
All the people needing IE6 for something could safely upgrade to a new OS, knowing their old web-apps would continue working for as long as LISAF is supported; and all the people developing for the web would be released from the burden of seeing IE6 outside an Intranet ever again.