"More resistant to scratches" is different then "Impervious to scratches". Blue-ray discs may have improved scratch protection, but over time they are still vulnerable. Another thing, I don't know which movies you rented in DVD, HD DVD and blueray, but blueray hasn't been around as long, thus the disks have likely been exposed to less (ab)use compared to DVDs. Furthermore, DVD players are still much more widely used, so there are proabaly a lot more people who would rent a DVD then a blueray, exposing the DVD to still more use.
As for flash drives, I know that they're expensive, but I'd much rather pay an extra dollar to rent a movie that I know isn't damaged then take my chances with an old DVD (or even blueray). They don't even have to be too compact, they're just playing a movie, no need to be the size of a quarter. Also, as much as I hate to rely on the future to back up my arguement, I'm sure that, given the initiative, somebody could come up with a less expensive flash drive movie system. Maybe ask Sandisk...?