What other software has the very first digit go up and the feature set go down? I'm surprised to hear you have this expectation, and I'm curious how wide-spread it is.
I think it arises from the way contrib modules are so intricately linked to major core versions. When PHP issues a new release, we don't (usually) have to release a new Drupal version to keep up with it. Since there's no backwards compatibility for contrib, it's a bit more complicated. So, when a module is ported, one of at least three to four situations might occur: 5.x-1.x branch gets ported to 6.x-1.x branch - just a straight port 5.x-1.x branch gets ported to 6.x-1.x branch - new features are added to the 6.x-1.x branch during this process 5.x-1.x branch gets a straight port to 6.x-1.x, and 5.x-2.x gets a straight port to 6.x-2.x 5.x-1.x branch is abandoned, users have to upgrade to 5.x-2.x then to 6.x-1.x (if the maintainer has dropped old updates, like core did in 5-6). So when looking at a 6.x-1.x (or 2.x) version of a module, you have no way of knowing whether it's a straight port, or a rewrite, or somewhere in-between. It's a bit like computer games - when they release a Grand Theft Auto game on a new platform, they don't call it GTA1 all over again ;). That said, I'm undecided on what the ideal path ought to be with this. Nat