I think it is time to start expecting some degree of conformity from web users. To this end, I would like to create a Basic Conformance Standard, according to which web servers could be configured to serve clients a page urging them to upgrade to a browser released in the last n years, for the sake of their own computer's security and the collective sanity of the world.
Write such a standard, giving implementation details, and the server modules will come implementing it. And then the web admins can make their choice. Elements of this are already in effect, but a standardized effort would be of great help.