At some point people need to be pushed off. That's all there is to it. A lot of wasted developer time goes into supporting old browsers. Probably a lot more time than it would take to upgrade the browsers in the first place. If a large portion of the Internet stops working because a company/govt is using IE7, Firefox 3.6, etc, maybe they'll come to the conclusion that it's worth looking at different options.
While it's tempting to use the threat of obsolescence to force governments and enterprises to adopt newer software, these groups are traditionally the most resistant to such efforts. This is due not in significantly to the bastions of old tech living in iso, where all they have to talk to is the custom-built application.
Also, the promise that the forced obsolescence will eventually work out better for the world is cold comfort when it's you who must choose between legacy tech and sacrificing a large portion of your revenue.
