Unfortunately, not all updates can be tons of new content for "vets who have mastered the entire game" to get tons of new gameplay out of. This update focused on fleshing out a biome which, until this point, was somewhat underwhelming. The majority of this content was aimed at the early hardmode stage, not exactly intended as a new attraction for end game characters. Taken as a whole, I believe it offers a pretty complete experience now, but these additions weren't exactly something that's going to provide months of gameplay like a major update (1.1, 1.2., 1.3, etc). For "old players", yes, that's how its going to feel, but that isn't the intention of these additions. One of the biggest pieces of feedback from 1.3 was "expand the desert/underground desert", and this did. Some people will always ask for more, but this update DID add a good bit of content to that section of the game.
As for that, not every enemy needs a drop, and not every enemy needed new drops. Sand Slimes, just as a really easy example, have very little reason to be given a drop . . . almost all slimes just drop gel anyway.
Sand Sharks drop Shark Fins, and alt variants drop light and dark shards, which, along with Mummies, make them the component source for two of the new items added to the update which are notably NOT tied to the new miniboss in any way. I'd say this is enough to sustain them as useful enemy.
I would have liked to see content additions for Dune Splicers and Sand Poachers, personally. But, again, not every enemy needs a drop, and Ghouls, Antlion Chargers, Lamias, Basilisks and Desert Spirits DID get new drops, so I think things balance out a bit better than your post would indicate. This doesn't mean I wouldn't want to revisit Dune Splicers/Sand Poachers at a later date, but its ultimately not my call with what ends up being added.
