Killing Phasma was fine. Finn’s arc was never about revenge. It was about finding his path outside of the empire. The obvious conclusions are (a) he fully integrates into the new rebellion, perhaps taking on a leadership role, or (b) he finds that he doesn’t gel there either and chooses a third path. Neither required a showy throw-down with Phasma.

It’s fully on RoS that they chose neither and did nothing with him.