You realise either way this disproves your point about Finn's characterisation, right?
So first off this statement "and NOT turned out force sensitive" is completely false. We have no confirmation on Finn's.....midichlorian count but there were hints toward him having force sensitivity. Regardless, there are two options. Either:
1. Finn is not force sensitive. He is then the only non-cyborg non-force user to have battled with a lightsaber in the films thus far. This speaks extremely highly of Finn's ability, to wield a lightsaber the way he did the first time using it, to land a blow, against a trained dark side user, against a Skywalker no less. This is very impressive.
2. Finn IS force sensitive. Again what he did is impressive, and it means the potential for him to become a Jedi or trained force user in the future. And of course this would make your whole non-issue of him using a lightsaber and not being force sensitive wrong.