OK, let's play this one out.

While a person is engaging in the mileage run itself - and let's assume some sort of extreme 0 to 100k mileage runner, going bonkers in January - he's purposely flying the least-desirable times and days for a business traveler. That's how he scored the MR-worthy fare. Probably bouncing around from hub to hub on a Saturday or whatnot. Or maybe flying internationally, taking upgrades out of the equation entirely.

So during the MR, nobody's getting displaced. If the flight was that full of elites to begin with, it wouldn't have had the MR-worthy fare.

Then, later in the year, if the mileage-runner wants to displace these other travelers, what does he have to do? He has to buy a ticket on that higher-demand flight, probably paying a higher yield himself in the process.

I think of it just like any other form of status comp, gift, fast-track, challenge, etc. Setting aside for a moment that there are probably 10,000 RSM's flown by "real" elites for every 1 flown by any kind of "lite" elite, the only way the "lite" elite can ever gain a benefit is by, you guessed it, buying a ticket and engaging with the brand.

In other words, although I'm not a mileage runner and never will be, I lose zero sleep at night that the miniscule cadre of people around the world who do this are ever going to wind up snagging the *last* available upgrade seat on my specific flight where I'm using a mid/low-tier elite status, relegating me to coach. It's the same amount of sleep I lose wondering if I'm going to win the Powerball.