Look to me it's more about having Ash lose to someone we at least know something about, Ritchie, Harrison and Tyson, even though Ash only met them when arriving at each respective league prior to sinnoh, they were given enough character to see them as more than just opponents but actual characters.
Tobias on the other hand, he was literally just there to keep Ash from winning, that's not a character at all, heck we've never even heard of him again since then, that's pretty telling considering that as much as people don't like Alain, he at least can be called an actual character(and he did return for the masters 8...though I'm sure people have made it known how to feel about that) and not a plot device.
Heck if Ash was at his supposed "prime" during the sinnoh league, then he should've won the thing, let alone use better pokemon than Gible or Torkoal against Tobias, those 2 had no business being in that match, and better tactics because opening with hyper beam against Darkrai was a huge mistake as well.
Conway was by no means a weak trainer, we saw he had other pokemon during the tag tournament with Dawn, again all I'm saying is that I would've rather had him be the one to take Ash down in the sinnoh league if Ash was to lose because i'd rather it be to someone that can actually be called a character.