Gary wasn't handled well either, but it made more sense to have a full battle with him than to have something like that for Trip. I'm not even sure if the Junior Cup could be counted as buildup for their battle since he was using just his overpowered Serperior, instead of using multiple Pokemon that could have appeared in a full battle, and I think that it was just so that Alder could cause Trip to change his attitude. He was written off badly, but it made sense with how poorly written his rivalry with Ash was. Even his rivalry with Gary was better by comparison, which says a lot in my opinion. I'm still perfectly fine with how Trip was written off since I think that they lost interest in him far more than they did with Gary, despite how he appeared infrequently, since there were other rival characters to focus on and they clearly preferred them more than Trip given how often they appeared by comparison.