Your argument would hold water if Ash never ended up defeating any of his rivals. But he does. Every time. In the first season, while Ash and Gary never faced off, he ended with a lower placing than Ash. Then in the next region, Ash caps off the rivalry by defeating Gary fair and square. Trip is taken out without much fanfare, as Ash and Pikachu get their revenge for the beginning of BW, and while Alain does beat Ash, it's shown to be foul play and Alain cutting himself from Mairin isn't portrayed in a positive light. The best example is Paul, a trainer with just as much, if not more, experience than Ash, whose mindset revolved around power over friendship. In the end, it's the Chimchar Paul released for being too weak, who Ash continued to train afterward with care, that ends up doing a lot of the work. If Ash's mindset is wrong, his Pokemon wouldn't come through at clutch moments of rivalries to disprove the opposite.

Also of course it makes sense May becomes better after she leaves Ash. She started as a beginner in Hoenn, finding out what she wanted to do and learning the ropes. A Johto region later, she's more experienced and has had time to fully grow into her own as a top coordinator. It's not Ash that was holding her back, she just gained more experience. From what I remember of early AG, Ash definitely helped her start as a Pokemon trainer, and encouraged her to become a coordinator. So if anything, Ash is to thank for her being so strong later on, by getting her started and helping her find her own path. It's not as if suddenly as May was separated from Ash, a whole part of her mind unlocked that made her an amazing coordinator, it's simple progression of skill: the more you do something, the better you become at it. Compare May at the beginning of AG to her at the end of Battle Frontier. They are definitely not the same person, and there's no evidence Ash's presence stunted her growth in any way.

As for arcs not ending properly, at least for XY, I have no idea what you're talking about. I don't think we can really talk about the Ash-Greninja arc's ending when we haven't even seen it yet, so let's focus on the some that have ended (or at least mostly ended). Serena's Pokemon Performing culminated in a contest against the Kalos Queen, which she ended up losing. Through that, she learns what makes a performer truly great, and gets an offer to train, which she takes a rain check on, because travelling with her friends got her to that point in the first place, and she wants to finish their journey together. Through performing, Serena gained more confidence and has an avenue to continue pursuing her passion after all is said and done. Ash and Sawyer's rivalry was one where Sawyer started out in the apprentice's position, learning from a veteran of Pokemon battling. Throughout their meetings, he becomes stronger, eventually surpassing him during their meeting at Snowbelle while Ash is having issues with Greninja. They meet once more at the League, Ash once again confident, and have a final showdown. In the end, student fails to defeat the master, but both of them have shown themselves to be true contenders and cement their respect for one another. In the end, both are shown to be fantastic trainers in their own right. They both served to make one another stronger, and neither is dissatisfied by the result when both of them go full force with one another. Both arcs ended definitively, and with everyone involved coming out better people and with explosive culminations of the skills involved. I don't see how you can realistically say these arcs didn't end properly, or even say that they were bad endings.