Your meme pertains to exponential /growth/, not exponential /decline/. Exponential growth in a population hits a limit as a population saturates, causing the sigmoid shape. Exponential decline hits a constant asymptote and bottoms out. For populations, that asymptote tends to be zero. For cost curves, as in the solar panel case, it will be something more like capital cost of production divided by panel lifetime, which can be a very small number indeed.