While all the bleeding edge Dotters here scream "Upgrade" I'd like to ask why? How many web pages are actually using the new capabilities of the upgraded browsers? And how many pages just changed one minor widget? I maintain there's _zero_ reason to tell people they can't view a page without upgrading. It's the old days of "Best if viewed with Internet Explorer" all over again. As you all say, disk drveis are getting huge and the old pages take almost zero space. Add note saying it's not supported and here's a link to the latest and greatest. People who want the new abilities will upgrade.
Remember, you're not the not the only provider on the net. If people only the choice of: 1) Change their entire web serving experience to increase the job security of yuor web developers, or 2) Leave your site, you and your shharreholders might not like the answer.