Rather, I think the main reason why Ash lost that one was because of his own stupidity, optimism and lack of knowledge on the opponent. When someone comes in with a Darkrai, it shouldn't be too surprising for that same trainer to have more legendary Pokemon or a team of over-the-top powerful regular Pokemon backing it up. By all rights, Ash had the means to counter Tobias's Pokemon, and even if he was focused on Darkrai alone, he could've just assumed the worst and brought some of his best Pokemon instead. We've already seen that Ash is capable of calling Charizard over all the way from Unova, but no, instead we saw 2 average Pokemon and a throwaway Pokemon get one-shotted by Darkrai alone. The battle itself could have been a callback to some of the previous techniques that Ash learned through his Sinnoh adventures (i.e., counter-shield from the battle with Fantina).
There could be some arguments made that if Ash had beat Tobias and won the league, the series would've been over, but if you ask me, it could've given the writers a chance to either truly start fresh and enforce the whole "new beginnings" theme, or delve into what Ash would be going through after beating the League and Champion in Sinnoh. Even if Ash beat the Champion, he would probably just give the title back to Cynthia and leave for a new adventure; also, being Champion does not mean you've become a Pokemon Master (vague dream as it is), so all this could make Ash begin to question what a Pokemon Master really is. In search for this answer, he could tackle the Unova league and the writers could've really hammered in his "mentor" role through Trip and maybe Iris while still keeping his personality intact.
Well, I've droned on for long enough. I can talk as much as I want about what would be ideal, but odds are, I bet no one's going to even bother to respond to this post.