As this lucid article in Slate describes the investment boom in alternative energy, which he describes as the Prius Bubble, is ". . . a process that has repeated itself throughout history—with the railroad and telegraph, for example—investment bubbles frequently kick-start new industries and leave behind innovations and commercial infrastructure that others can use.."

It was certainly true of the Telecoms industry, which almost invested itself out of business by laying fiberoptic cables all over the world, lowering the cost of conventional telecommunications irreversibly and pricing in new web-based telephony.

I'm not entirely convinced that you can apply this to existing alternative energy assets, but do quite agree that there is a known unknown - we don't know what technological breakthroughs are going to come out of this investment boom. The argument makes more sense if you look at alternative energy as a forward purchase at a fixed price - decades into the future with say a wind farm or solar panels. If you take the view that conventional energy prices are liable to rise faster than incomes for the next couple of decades - then it is quite possible that the typical payback period gets that much closer. Renewable energy prices can be fixed into the future of course, because the cost of the fuel - wind, sun, etc. - is free.