Because I don't believe Prius users are the kind of folks that will be dumping their cars en masse to buy gas guzzlers just because of a temporary price drop, I don't subscribe to the theory.
More plausible is a better market for used cars, overall, starting in about three years -- as the increasing supply of new cars begins to hit the used market.
From the Great Recession, annual U.S. new car sales are now up -- what? -- close to 70%? Assuming 25% of cars sold are leased PLUS typical owner fatigue after 5 years of ownership -- that alone could change the supply/demand construct of the used market.