To be honest, the difference between HD formats and "regular" DVDs is NOT as dramatic as the one between VHS and DVD.
I was just talking to a friend of mine about this, and we both agreed - the fact that the DVD brought so many innovations to the average consumer (widescreen, DD, DTS, loads of extras) made it a very desirable medium... the new formats don't offer that much of a leap, even if if the consumers have bought the new LCD or plasma TV sets, but the average screen size is still under 42 inch...
Honestly, if you play DVD through a good upconverting machine (such as an Oppo player, or a HTPC) you will not be seeing much of a difference between that and HD formats.
Along the same lines, remember when people said dixv and xvid look just as good as the original DVDs? I used to laugh at these statements, but I have to admit that I've seen some pretty impressive .avi's and .mkv's lately, as the encoding algorithms just got better and better.
There's still life left in DVDs, and the fact that the format is mature, stable and so much versatile than its successors (especially since copy protection and region coding have become irrelevant) will ensure its longevity.