BluRay is superior in many ways. Just not enough ways. It's not like VHS to DVD. It's just a bigger, sharper image version of DVD. It's DVD+, honestly. And to me, as someone who already owns a huge DVD collection, I just see no super compelling reason to suddenly switch mediums when the medium I've been buying for two decades now, is perfectly fine.
I don't know that I believe DVD will ever be "overtaken" by BluRay, and as someone who appreciates having physical copies of films, not just digital ones, I personally hope it sticks around for quite some time.