Is authorization done with some activeX-applet? Otherwise I don't really see what IE is offering that Chrome offers too, but scrapping-libs can't.
BTW There are libs to use chrome programatically for those things. It's called headless-mode or chromeDriver. Similar things exist for other Browsers. So you could probably rewrite your app to become more indipendant without lossing the existing functionality.
I just don't see enough cost benefit from rewriting hundreds of thousands of lines of working code at this time. I can't see IE going away, any time soon, due to corporate tie-in. Hopefully VBScript ditto. If it all falls over down the track, I'll review my options then :-)
BTW There are libs to use chrome programatically for those things. It's called headless-mode or chromeDriver. Similar things exist for other Browsers. So you could probably rewrite your app to become more indipendant without lossing the existing functionality.