I'm just drunkenly stumbling in and ignoring the rest of the thread to answer the actual question being asked of this thread because I watched the entire Sinnoh League this morning. What went wrong is that we don't know Tobias's story. It's as simple as that. We don't know how he met Darkrai and Latios. We don't know how he managed to convince them to stay inside PokéBalls. We don't know if they're somehow his first Pokémon. We don't know if he has exclusively Legendary Pokémon, or if he has some non-Legendary partners in reserve. Judging from how he says that a full-power Draco Meteor from Ash's Gible would have one-shot any of his non-Darkrai Pokémon, I'm guessing he's got to have some non-Legendary Dragons in reserve, but we don't know.
Ritchie is immediately established as a mirror image of Ash, and gets to bond with him by rescuing Pokémon from Team Rocket. Harrison has Blaziken, a powerful new Pokémon from Hoenn, and Ash helps him catch a Sneasel. Tyson gets some time to hang with Ash in-between matches and explains his Meowth's backstory over time. Alan got four Mega Evolution Specials, a couple of interactions with Ash, and still had an unresolved character arc that needed to be completed. Even Cameron got an episode dedicated to battling Marlon for his 8th badge to establish that he's not a pushover, despite his scatterbrained and idiotic nature. That's what helps make every other battle that knocks Ash out of Pokémon Leagues sting less. There was some kind of set-up for who these characters are. We understand who they are and can connect with them, so we're able to more readily accept it when they shatter Ash's hopes and dreams. The only faux pas made with Alan is that the anime very intentionally overhyped the battle and got fans on board with the idea of Ash winning so that more people would watch the League. And the faux pas with Cameron was playing up the idiot part too hard, thus making it implausible for him to believably beat Ash.
Tobias doesn't get any of that. Barry passes him in the hallway in-between matches and says "this crazy strong Trainer swept his opponent with a Darkrai". Conway says "not only has he won all of his matches with just a Darkrai. He won all of his Gym battles using just Darkrai too". Then we don't hear about Tobias again until Ash is paired off with him, and we jump right into Ash and Tobias battling after that. There's no story there! It's just a mysterious and strong guy coming out of nowhere! What we needed was an extra episode where Tobias explains how he managed to befriend Darkrai, maybe establish that it's the same Darkrai Ash and the gang saw earlier in the series so there's more of a connection there. Then, Tobias's battle with Ash needed to be a two-parter to explain how he got Latios as well when he sent it out.
The battle can be the exact same, but there's at least a story there now to connect us to Tobias so we're more receptive to him winning over Ash. ...But they didn't do any of that, so we got one of the worst "rival-out-of-nowhere" characters ever instead.