Years ago, people said renewables wouldn't ever amount to much. Sure, it's growing exponentially, but look at how small it is! And yes, costs keep going down but it will take forever before it's competitive with coal and nuclear.
Forever's come and gone. Oil and gas went up in price. Exponential growth generated a nice experience curve http://en.wikipedia.org/wiki/Experience_curve_effects
Now some critics and skeptics will go around saying no one could have predicted how quickly wind and solar took over. They'll say it was due to the rapid increase in oil prices (no one saw that coming either) or subsidies (German feed-in tariffs were very generous, though not as much as tax breaks for oil co's).
So the lesson: humans are really bad at forecasting the results exponential growth. If you can find one that people are ignoring and time it well, you can entrench yourself in a market before most people even realize it exists.