That is in fact the only way they could maintain it. They are used burstily and have very little use for a standing fleet doing nothing for most of the day.

That would only work if supply and demand spiked at the same time, and I doubt that would be the case.

I suppose surge pricing and whatnot address that to a point, but if every Uber driver is scheduling their day around Uber demand, then that's hardly a sidegig.