What they need to do is knock down rebels' expose on basic chance by 10 or 20% instead of being guaranteed.
Nerfing finns ability seems counter to the whole idea of making 3p0 promote cross faction squads
Treyas tenacity is high enough that you need a real lot of it, so your team won't have crazy offense or speed. I ran the team with 50~80% potency and held the first topple for quite a while, but only did ~2mil. Damage.
Cutting down the expose chance by just a little bit would break the infinite loop cycle, keep old toons unchanged, and also keep Finn+rebel team viable for STR raid. I don't think it would hurt pvp modes much, if at all, and overall I think it would let 3p0 maintain more value than if they just buff STR/Nerf finn