...it could require a fairly heavy overhaul for modern browsers.
That's absolutely correct. I'm currently working on a web application at the corporation I work for. It's been so badly coded (long before I arrived) that making it compatible with IE7 (not to mention Firefox or any other browsers) would be a nightmare that would probably take our development team a year to complete. And I'm not certain I entirely agree with you on the interface point. I think major interface changes between versions of a program are huge deterrents to upgrading for many people.