Doublebooking practice is simply unsustainable for vendors - the end result is longer cancellation deadlines, higher fees etc - just look at hotels and airlines.
It's just a matter of time before this comes up in rental car segment.

I like national so i dont want to make it harder for them by making unnecessary bookings - their policies are much better than those of alternatives and i really hope they will last as long as possible...
OTOH i would pretty much consider anything as a fair game with hertz - obviously this will be YMMV kind of thing