Yep. This discussion has come up a number of times, and many people like myself will say that it's not a good idea to hoard miles. The unredeemed miles vs. seats problem has been well-known for years, and at least when it comes to the largest U.S. carriers there's not even a credible attempt to maintain a balance. Even as they get stingy about RDMs for flying they'll still mint them like crazy via credit cards.

I've earned just under 3M RDMs lifetime but might have about 250K left between all programs, with the highest amount retained around 420K at one point.

One problem is that earning is very unevenly distributed. In workplaces you usually have people who travel for work way too much, and those who hardly do at all. Many people with hefty mileage balances from work will tell you that they only have limited chances to redeem on vacations (as those are ridiculously stingy in the U.S. compared to the rest of the developed world) and are saving the pile for retirement when they'll have more time. Because the baby boom is now retiring you have the demographic bulge, adding to demand and pressure for huge devaluations like AA's recent ones with F and J (and Y).

It's a bit like depositing drachmas in a 0.5% interest account in pre-euro Greece. You just know the currency will lose value to inflation if it's not occasionally outright devalued, and it doesn't gain anything by sitting there.