So, the Marines are burning the Harriers and Hornets like Cortes buring his ships at Veracruz ... the only way is forward.
If the F-35 program managers are on top of their procurement game, they'll have the Harrier and Hornets destroyed rather than mothballed in the desert.
If the Harrier and Hornet manufacturers and servicing companies have their hooks still in the Marine procurement bureaucracy, they'll sell the planes off to other airforces to keep generating the maintenance revenue from those old birds.
If the old birds are mothballed in the desert, there's always the risk they may be brought back if the F-35's experience growing pains or burn-in issues. The F-35 program manager, possibly looking towards future employment at the F-35 manaufacturer, will want to make sure the Marines can only move forward with F-35's, and not re-call the old birds to service.
Put all your eggs in one basket ... what could possibly go wrong? Just ask Boeing airliner customers.