The main factor that these and other equations miss is that AA miles are a poor store of long-term value. The US dollar might also be a poor store of value if government spending policies continue in this manner, but historically, the US dollar has been a much better store of value than AA miles.

With that said, I have put a large amount of cash in BankDirect AA accounts. Obviously, I am willing to gamble on AA's creditworthiness to get a slightly better return on my cash. However, it's not a trade that is necessarily suitable for everyone, and it's a closer comparison than it appears at first glance, particularly if you are talking about accumulating a ton of miles that you might not use for years. This definitely applies to everyone using BD to accumulate 1mm or 2mm miles for lifetime status.

If the miles do not get used for a few years then there is always a chance for a miles devaluation of 50% or more. Even worse, AA is a terrible credit (I recently looked at their financials and I used to be a credit analyst). It would not surprise me at all if AA had to file for bankruptcy in a few years. Many carriers have preserved their customer mileage accounts through bankruptcy, so we may be ok, but there are never guarantees in bankruptcy. If the carrier liquidates because the operations are so unprofitable that it is not even worth operating, then your effective miles devaluation is 100%! I don't think that would happen to AA but the airline industry is terrible competitive, and if major routes become unprofitable (because of $200/bbl oil prices or whatever) then that is definitely in the realm of possibility.