One factor which doesn't appear to have been mentioned in the thread is the ability of airlines to effectively have a throttle on mileage earning activity by requiring the passenger to be on the aircraft to earn miles. If this wasn't the case it would be feasible to earn elite status within a day or two just by trawling for acceptable fares and purchasing them. Even without elite status, with some of the very good fares available which allow destination miles to be earned for less than accepted redemption value this would effectively be fare arbitrage.
There are the obvious limits on the number of available reward seats and so on which would limit the impact of this, but the airlines certainly don't need a group (which would likely grow quickly once word got out about just how little effort was involved - a key factor which probably keeps the MR group relatively small) earning very large amounts of miles in very short spaces of time! Outstanding frequent flier miles and their potential impact to revenue do have to be included in the accounts so that could stand as one business reason why passengers are required to fly to earn the miles.

Very much a newbie here, so please be kind with the replies