Keep in mind that the national free day credits are good for one ways... and also keep in mind that if you have Exec Elite status (not just Exec), the guaranteed availability is really excellent. There's also a rate cap with Exec Elite status... I've been able to get cars for $90 in Maui or whatever when others reported $400/day or sold out.

So even though you're theoretically giving up 7500 AA miles per national free day (6250 AA miles if you have Exec Elite, where it's only 5 credits per free day), that's worthwhile for me. I've had some one way rentals (think, Suburban one way from Buffalo to Toronto during covid) that would have been $600 but that were covered by the free day.