The authors present an interesting description of the profile of: resale car prices, maintenance costs and rental rates, to argue that flat rental is puzzling. More precisely, they argue that firms
are behaving suboptimally by not keeping their fleets longer, and renting older units at a cheaper rate.

I do not find the argument (that there is a puzzle) convincing. There are companies that specialize in renting used cars. Indeed, there are not many firms in that niche of the market and these firms
are not well known. Is this puzzling or evidence that there is not much demand for older and less reliable units?

The authors do not seem to have the necessary data to make the case. There are operational costs (maintenance is only part of them) as well as reputation costs due to reliability. What about car
manufacturers? Rental fleets are a major form of advertising for manufacturers. The last thing they want is unreliable units of their brand in the hands of potential buyers. What about the
cannibalization of profits on newer units (of same or lower quality)?

I am not sure why dynamics are needed. Perhaps they are, but the way the exercise is described it seems like purely accounting (of the costs and benefits of holding units for an extra period).