But the only reason that is true is because they consciously left those new features out of the old interface. The improvements for moderators could be added to the original layout just the same.
For actually using the site the old design is about 100x better in every way except for mindless scrolling.
Sure. But to be fair, that is true with most new versions of any software... the developers could always commit resources to adding the new features to the old version too, but they're trying to migrate users to the new version so they don't have to commit resources to two versions.
It was less the features or lack there-of that drove Digg users away with v4, and more the change in culture around news websites being "official" and auto-submitting their stories.
That's not entirely true... the redesign has some moderation and content submission improvements. But, if you don't mod or submit content then you might not notice those improvements.
That said, like digg, the redesign is also missing a lot of features that old.reddit has. So, if they don't want to digg v4 themselves (which is what made Reddit popular to begin with) then they have to at least keep old.reddit around until there is feature parity.
They will definitely shed a lot of users when they drop old.reddit but anyone who uses particular subreddits for their hobbies and interests may bite the bullet and get used to the redesign if they like the community enough. Hopefully Reddit Enhancement Suite makes a lot of improvements when that happens because RES makes reddit so much better.