Unused PTO must be paid out in many states and in states where it is not legally required it is generally still required if the business has historically done it. Beyond that, Southwest pilots are unionized. No sizable union would allow separation without PTO payout.
Unions are for people in the Union, aka the people still working there, not for those getting fired with cause (failure to get vaccinated). This sounds exactly like something a union wouldn't care about.
What are your credentials for telling us things that unions do and don’t care about? What unions are you in? What direct experience do you have with how unions decide what’s important to them?