Can We Speed Up Browser Evolution?
In Can We Speed Up Browser Evolution?, Mike Davidson has some interesting ideas on how adoption of modern web browsers can be sped up. The discussion in the comments is also very interesting.
As for the argument about denying access to users of certain ancient browsers, yes, in most cases the site owner can make that choice. However, I would definitely not recommend doing so. Just let everybody in. No need for workarounds or light styling. Let everybody access the (well-structured) contents of the site.