I suppose that is possible, but consumer gas consumption is mostly non-elastic (i.e. does not change with the price). We saw this during the price spikes a few years ago. It doesn't strike me as likely that it would suddenly get elastic when moving downwards.

This is assuming for the sake of argument that hybrids are cheaper per mile. The thing that makes that claim difficult to evaluate is "Cheaper than what". There are several plausible candidates for a yardstick which make the claim catastrophically false. (Gas is a fraction of the cost per mile. People notice it because they don't remember ever not having a $350 car payment and don't consider the cost-per-mile that was rolled into that.)