I agree they didn’t know what to do with Finn. I think the bigger issue for Boyega is that there actually would have been enough screen time to do something interesting with
Finn‘s character, they just squandered the opportunity.
Look how in TLJ, they had Finn sort of just running errands, almost starting a relationship with Rose, almost being confronted with his First Order past.
And then in RoS, they had his character on pause trying to tell Rey... something (maybe about being Force-sensitive?).
Why not have him train and become a Jedi (or a new category of people who develop Force-sensitivity)? Why not have him (as others have mentioned) struggle with his past, become a baddie for a while, or even be part of a love triangle? These things would have made the character worth including, but they ended up not doing much with him at all.