As an example, let's say you're a regular business traveler flying a $5k business class round-trip from NYC to London on American Airlines. You have the highest status level which gives you a 120% bonus on miles earned. Each dollar spent on the flight thus earns you 11 miles, so that one round-trip is worth 55k miles. Source: https://www.aa.com/i18n/aadvantage-program/miles/aadvantage-...
But wait, there's more. Of course you also bought the $5k flight on your AA credit card, which gives another 2 miles per dollar spent, which puts the total up to 65k miles earned for one 7k mile round-trip flight. Or maybe you put it on your Chase Sapphire Reserve card, which gives you 3X Chase Ultimate Rewards points for travel spending, so 55k AA miles + 15k UR points. Now imagine that you're doing this trip several times per month and the miles/points add up very quickly; you can get to a million in no time.
American Airlines, for example, only gives you gold status (the lowest level) for life at 1M miles.
With status multipliers (but flying meat space), the most I ever hit was 623k miles in a year. I suspect it was close to 300k 'real' miles. Our travel person was booking 'round the world' tickets to save money at that point.
They devalue existing miles but remain competitive going forward by requiring, and giving, more miles for the same trip.
Miles? Ultimately not that many -- or not enough to be of long term value. It was enough to get high enough in the status tiers for reliable upgrades, but for the most part, a few ad-hoc spontaneous flights and that giant bank of miles you built up over the years evaporates over night.
It's a weird carrot on the stick that you kind of fall into. To ensure status, I would take purposefully crappy flights just to rack up the miles.
Looking back it was a silly thing to chase and a really stupid use of time.
After going through that, and playing the airmiles game, airline miles seem to only exist to trigger human emotions around the fear of losing out and the same kind of dopamine dependency that's exploited by grindy video games and gambling. You need 5million experience to level up, and dammit, it feels so good when it rolls over.