I don’t see why Kylo/Ben’s story should matter only in respect to Rey’s. Even if you say Finn’s story in TLJ is supporting, he’s still having a story of his own with its own meaning, separate from Rey’s. These movies have never been about just one character’s story to the exclusion of another.

That said, Kylo’s story is important in respect to Rey’s. They’re supposed to compare and contrast with each other. Ultimately JJ took the mirroring too far by literally just giving them the same story in TROS -overcoming their bloodline. But it’s not supposed to be about bloodline as much as it is legacy. Rey’s story is charting her own legacy when there’s no predetermined path for her. Kylo/Ben spends the whole trilogy in the shadow of his legacy, trying to snuff out the Solo in him in TFA and the Skywalker in him in TLJ. Kylo’s struggle is overcoming the predetermined path, good or bad. His solution in TLJ is an overcorrection - destroy it all, the Jedi and the Sith. In TROS we should expect a progression for the character, a change. If we are shown in TLJ that his motto “kill the past” is wrong and leads to desolation, in TROS we should see him grow and come to understand that there is a healthy way to integrate the past into his life. To me, the natural conclusion is for him to come to terms with his legacy and accept it for what it is after running away from it for two movies.

This is where Rey comes in. If her story in TROS is supposed to be using the knowledge of the past to build something new, then the two should naturally intertwine, for real this time (their team up in TLJ being a tease of what was to come, but not a real union because ideologically they were on two different pages).

After TFA, sure I believed Kylo would be redeemed just because of the simple “because that’s Star Wars” reason. But after TLJ, I knew Kylo would be redeemed because that was the only conclusion that made sense for the character as had been developed.