Can someone just complete their PPL and opt out of the program?
I suppose so, but if your goal is just to get a PPL then I would recommend going to a local flight school.
The pricing for the Airline Career Program is based on earning all the ratings in the program. If you “opt out” after a private rating you are charged an hourly rate for the 80 hours flown. The cost in this situation would be prohibitive and it’s recomended if you do not intend to complete the entire program you should earn the rating outside of ATP.