Maybe go back and watch Arman vs Islam, the first 1 rounds for tit for tat offensively and defensively for both of them, and that was Arman at 22 years old on short notice in his debut. Furthermore, Islam has never wrestled someone in his MMA career with as good of defense as Damir Ismagulov the way Arman did. The only real difference is Islam's cardio has looked considerably better than Armans who is in my opinion the divisions 2nd best Wrestler in the UFC with Gamrot probably being 3rd.
I think it's more common fighters have certain holes or limits to their game that never get filled or expanded upon. Usually guys still improve a bit more from such an early age even if it's just IQ, fluidity, sharpening technique, composure etc. But yeah, Chandler peaked early and has just stagnated since. I think in part because he didn't need to get better, Eddie Alvarez is to this day the best fighter Chandler has ever beaten and probably the 2nd best fighter he's ever faced (its close between Alvarez and Poirier, Bendo was too out of his prime). And Chandlers style has always relied on his athleticism and him being the hammer.