I think any retail chain is going to have a hard time convincing "soccer moms" or any non-enthusiast group to upgrade to HD-DVD. With the move from VHS to DVD, the new format offered a more convenient medium, extra features, scene selection, ability to skip previews...etc. What does HD-DVD have? A better picture?
A significantly better picture.
But yeah, if picture means nothing to the buyer, then there's no reason to upgrade.