Well, I'd definitely blame random fillers, Team Rocket, and Team Flare. Ash's companions having their own goals can sometimes derail Ash's pursuits, but it's also worth noting that he's grown stronger in part due to their presence as well. Citron is currently assisting in analyzing the Ash Greninja phenomenon, Serena has reassured Ash on multiple occasions and is there to support him when he's reckless or puts himself in harm's way, etc. So I do think that's less of an issue than everything else. Ash can be a bit narrow-minded at times when he goes too far down a certain path (be it "I want to get stronger", or anything else). His companions help keep him grounded, provide occasional training partners, and often help him strive to grow stronger to protect them.
I didn't necessarily mean that they were bad news and Ash should avoid having traveling companions with their own goals.
I'm just saying that they do sometimes divert attention from Ash's goal to theirs.
Again I'm not suggesting fault here, I'm not speaking realistically here. That while I'm sure Ash is better off with traveling companions, Ash might be able to accomplish things faster and more of it, if he didn't.
Of course I'm not saying it needs or should be changed. But to realize because of it, other trainers (rivals of Ash), Sawyer in particular are not hindered in such a way and could realistically grow faster than Ash can grow.
The issue with Sawyer though is that we're not seeing any onscreen success for it to be justified to Ash losing him in the league. Instead any success Sawyer does have is always something we don't see.
As I said before, because of the hindrance to Ash's goal, Sawyer can believably close the gap between Ash and himself. My issue comes from we're not seeing this gap close when they battle except for Sceptile, and then to suddenly be okay with Ash's other Pokemon struggling against Sawyer, or even Sawyer manages to beat Ash in the league, THAT is what's unbelievable.
If Ash didn't basically one shot Shelgon with Hawlucha, one shot Honedge with Pikachu and one shot Sceptile with Ash Greninja, I would be FAR more willing to accept Sawyer defeating Ash. Instead Sawyer is one shotted multiple times, hell Pokemon fan says as much Sceptile was one shotted by Ash Greninja (if I'm not mistaken).
So to see him only after 10 episodes being close in strength to Ash, with 3 badges including Wulfric's badge, a badge Ash is apparently struggling to get, doesn't feel justified. I've no problem with closing the gap for the league, and so it can be a down to the wire battle. But it should always still end with Ash being victorious.
But Pokemon isn't know for being realistic, so why bother thinking they would.