Cordelli, your logic sucks... If when I showed up at the Ritz Berlin today (after 3pm) and I was told no rooms were available but a suite or club room (typically cost a couple hundred Euro more) and they told me to pay the price difference or wait until someone with a late checkout left and had the room cleaned I would be floored. As I am sure you would. (I got the club anyway)
I book a car if it isn't there they better give me whatever they have (They can attempt to up-sell but that's it) Up-selling shows that the MBZ isn't even on reserve. At the end of the day it's another car.. Nothing special at that. In MUC the other day my reserved MBZ C class wasn't available, so I got a new hell of a lot better Beamer no problem. (OK I know this isn't 'mercah here) Nonetheless, you book something with your credit card, and expect it you should get whatever they have, it isn't your fault they don't have the car.
I hope one day you find yourself in the same situation lacking the funds to upgrade, and have to wait.. Maybe even call off the event you are attending. Sorry to be so blunt about this, I just find it disgusting, and I know how crappy some Hertz employees are.