There've been dramatic reversals in the price of oil over the past couple of decades, most triggered by geopolitical developments and at least one by a technological development (i.e. fracking).  We haven't seen the last reversal nor we can predict either its date, cause or magnitude.

It follows that good public policy (quite apart from concerns about the environment or meeting Canada's Paris commitments) suggests that maintenance of ambitious motor vehicle fuel reduction targets should remain a constant goal?

Achievement of mass production of reasonably priced vehicles with significantly greater efficiency takes several years of hard work by the motor vehicle manufacturers (especially those in North America) and history shows that they are more than inclined to drag their heals unless legally obligated to act.