It may not be frail, but it's certainly not bulky either. Even though it resists Bullet Punch and Extremespeed, priority will still put a large dent in him every time. He's also weak to Aqua Jet, which means many rain teams can check him even in sand.
His bulk is similar to Flygon's, which is pretty dang good for such a fast and powerful sweeper. His Steel Typing also gives him a multitude of resists, meaning opportunities to come in and potentially set up. Unlike many typical frail sweepers, he can take strong neutral STAB attacks from defensive Pokemon and set up with health to spare. Also, Azumarill, Kabutops, and potentially Abagoora are virtually the only viable users of Aqua Jet, and a Rain Dance team is filled with Swift Swimmers who have no need for priority because of the auto speed boost they get. Again, Abagoora is pretty much the only one you'd see, and even then if it's a Rain Dance Shell Breaker it won't even need to carry Aqua Jet. Kabutops otoh, needs to choose between Aqua Jet and Low Kick. The other priority he is weak to, fighting, has helped the popularize the Mach Punchers Roopushin and Breloom to serve as checks to Doryuuzu. Thats an example of shaping the metagame right there.
Hes, he gets those moves and what else? Swords Dance? That's two usable sets. Claw Sharpen, Brick Break and Shadow Claw are there, but hardly worth mentioning. No priority. Nothing out of the ordinary. That means extremely predictable. And yes, it's strong, but it can be handled if you have the right tools.
Earthquake, Rock Slide, and Swords Dance is all he really needs to be a major threat. He has a multitude of options in the last slot, or even two more if one chooses to forgo SD; Return, X-Scissor, Brick Break, Rapid Spin, and Shadow Claw. Doryuuzu may be relatively predictable due to the effectiveness of it's typical set, but it is not a one-dimensional team member at all. Doryuuzu can act as an exceptional Sweeper, Revenge Killer, Rapid Spinner, and a Check to many threats, particularly with Balloon-even Garchomp for example. Doryuuzu also is, along with Starmie, the best offensive rapid spinner in the game.
Really? Tyranitar and Hippowdon can get to level 103? Gosh, nobody told me.
Both Ninetales and Politoed have very good tools as leads. Hypnosis, good STAB, Politoed has encore and can sponge special hits. The only thing he's missing is reliable recovery. So yes. They were NU for a reason, just as every pokemon has a tier for a reason. But before there were better options. Now, Toed and Tales can have their time in the sun (...or rain)
Dory is completely dependant on Sand. He is a huge threat, but to the same extent that Kingdra is in Rain. With this reliance on weather and some pretty abusable weaknesses, I do not believe he defines the generation by any stretch of the imagination.
The point is that even though their new abilities give Politoad and Ninetales a huge buff, Hippowdon and Tyranitar are still much, much better Pokemon, and they aren't reliant on having their weather going to be useful either. Sandstorm tends to be the most balanced weather, consisting of primarily Ground, Rock, and Steel types. Rain teams consist mainly of water types and can have their weaknesses exploited much easier while sun teams tend to lack focus from what I've seen of them (I haven't fought many sun teams, but they have never been a threat so far).
Politoad and Ninetales' primary purpose is to provide team support, while Hippowdon and Tyranitar just happen to provide it while also primarily fulfilling other, more individual roles (a variety of different roles in Tyranitars case) that contribute to the team.
I also agree to an extent with you're comparison of Kingdra and Doryuuzu as weather abusers. The main differences is Kingdra has more survivability while Doryuuzu has more power. But the important difference is that, so far, Sandstorm is still the dominant weather, and the most common weather, in part because it's auto-setters are such great Pokemon themselves, meaning Doryuuzu will almost always have the Sandstorm it wants. I'm not saying that Doryuuzu "defines the generation" at all. It's much too early for such statements. However, I do believe that he is easily the most dominant Pokemon in the game right now, and one of the absolute best endgame sweepers that exists, Dream World or not.