The problem is the character never had any room to grow. When the protagonist defeats the villain in the first movie, where do you go from there? In Star Wars, Luke runs away from Vader in the hanger bay. In Empire, he scores a few small hits, but gets his ass kicked, and in Jedi he matures enough to hold back his anger and not go down the same path his father did. Ray feels like her arc is a straight line.
I remember back when TFA came out, most of the criticism I’d hear going around was on how it differed from the OT, rather than how similar it was, and one of such criticisms was how it moved “the duel” forward instead of saving it for the next movies like with Luke and Vader. Another common complaint I heard was how Kylo Ren was too immature and wasn’t intimidating enough to fit the new “Vader” role, despite that clearly being a conscious choice for his character.
In that regard, I think the duel at the end of TFA is great because it didn’t just propel Rey’s character forward, but Kylo’s, and I think TLJ did a great job following that thread, showing that his defeat rocked the very foundation of his beliefs. As for Rey, the duel marks the moment she finally embraces her calling, something Luke did way earlier in ANH, and I think her journey in TLJ became more interesting for it as it wasn’t just about becoming strong enough to confront Kylo.