Finn had a clear arc in The Last Jedi, and the clever thing was it was inverted from Rose’s even though the same experiences teach them the lessons. Rose and Poe’s arcs are to go from an “at all costs” approach, ready to sacrifice everything, to understanding that sometimes doing whatever it takes isn’t the same thing as doing it at all costs. Finn’s arc goes from a man without a cause, driven by fear, to becoming someone that understands there are things worth fighting for, even being ready to sacrifice himself. A lot of people say his arc is ruined since he doesn’t sacrifice himself, but that’s not true since he was actually ready to sacrifice himself. He was still making that choice.
In fact, him being stopped from sacrificing himself is part of Rose’s arc, with Poe agreeing with her as part of his. They’ve learned that reckless sacrifice isn’t always the solution.