When the Transition vision began, energy prices were about 60$/barrel and rising. There was an expectation that cheap oil supplies would start to run out soon. So as well as needing to wean ourselves off fossil fuels because of climate change we would have to cope with a big rise in transport costs. In fact that did start to happen Ė see the chart below. But the world economy kept going.
In 2014 oil prices plummeted because supply grew faster than demand, and they are still low but that isnít any reason to be complacent. We now know we have to leave two thirds of our fossil fuels in the ground in order to mitigate climate change - including a third of our oil. http://www.bartlett.ucl.ac.uk/sustainable/sustainable-news/nature_fossil_fuels So it isnít that we canít afford to produce oil for transport any more Ė we just canít afford to use so much of it.
On the other hand, the balance of supply and demand, and hence price, is a matter of international politics as well as economics and can change rapidly. When oil prices were high supplies were relatively secure because there were a large number of production fields all over the world. At lower prices most of those fields are unprofitable and we could end up relying mainly on OPEC countries again, because those fields can produce oil very cheaply. In fact, as we reduce demand for oil, lower prices mean supplies become more and more concentrated in just a few countries - and hence prices are even more vulnerable to manipulation. We need to be ready for anything.