The first time I lost real money on airline miles I was sitting in a chain coffee shop in San Francisco waiting for a call. I opened the app to price the flight I'd been saving for — Newark to Rome in the summer, one-way in the front cabin, a redemption I'd been quietly aiming at for two years. The number had moved. Not by a little. The price on the day I checked was ninety percent higher than the price I'd screenshotted eleven months earlier. Same route. Same cabin. Same airline. Different chart. No email had warned me. There was no email to send — the airline hadn't done anything wrong, in the sense that its terms let it change the chart whenever it liked, and it had liked to.
The word for this is devaluation, and if you fly with airline miles long enough it will happen to you. It has happened to every airline I've ever held miles with, at least twice. It happened during a pandemic when nobody was flying and it happened during a boom when everyone was. It happens when a carrier retires an award chart and replaces it with 'dynamic pricing,' which is a phrase that means the price is whatever the software says today. It happens when a carrier stays on a published chart and just quietly revises the chart on a Tuesday. The one thing devaluation never does is give you notice.
This is the central problem with single-airline miles, and it is the reason I have not seriously accumulated any for the last five years. When you earn a mile that lives on one carrier's ledger, you are — in the boring accounting sense — a creditor of that carrier. The carrier owes you a seat, at a price it publishes and can revise. You have no leverage on the revision. You cannot move your mile to a competitor whose chart didn't change. You are, in the sense that matters, hostage to the health of one company's loyalty economics for as long as you hold the balance. Balances feel weightless until the day the airline decides to weigh them differently.
Flexible currencies solve exactly this problem, and only this problem, and it is enough. When you earn a flexible point — Amex Membership Rewards, Chase Ultimate Rewards, Capital One Miles, Citi ThankYou, Bilt — you don't have a claim on a specific carrier. You have an option, deferred, on whichever of the currency's transfer partners still has a chart worth transferring into. If one airline devalues, you transfer somewhere else. If two devalue, you transfer to the third. You are not perfectly protected — every currency has bad transfer days, every transfer partner has occasional promotions and occasional cuts — but you have the one thing single-airline miles can't give you, which is the ability to route around the specific carrier that just made your redemption worse.
A currency you can move is a currency you can protect. A currency you can't move is a currency that only spends where its owner tells it to.
The specific example that made this concrete for me was a business-class seat on ANA to Tokyo. ANA is a Japanese carrier and its miles are difficult to earn in the US — there's no big co-brand card, the earn rates are modest, the accrual is slow. The seat is famously good; the award price on ANA's own chart is famously reasonable. The way people book it, almost always, is by transferring flexible currency into Virgin Atlantic, whose partnership with ANA lets them sell the same seat off Virgin's chart at a price that has stayed roughly stable for years while ANA's own chart has drifted. This is not a clever hack. It is the flexible-currency insurance policy paying out, quietly, on a redemption the airline itself would rather you not know how to make.
The counter-argument for airline miles is real and it has one strong form: if you fly one carrier a lot, its co-brand card will accelerate mile-earning on that carrier's flights, and the perks that come with it — free checked bags, priority boarding, an occasional companion certificate — pay for themselves inside a normal year. That is genuinely true. If you are a Delta loyalist because your home airport is Atlanta, or a United loyalist because your home airport is Newark, or a Southwest loyalist because your home airport is Denver, the co-brand card is a fine product to hold. What it should not be is your primary points-earning engine. Earn the miles you need on that airline to unlock the perks; run everything else through a flexible currency. The perks are stable. The miles beyond the perks are not.
The mistake I made for years — and see friends make now — is treating the co-brand mile and the flexible point as the same category of asset. They aren't. The co-brand mile is a claim on a shrinking asset. The flexible point is an option on a portfolio. Options are worth more than claims when the underlying is volatile, and airline loyalty programs are, by construction, volatile. This is why the same handful of transfer partners keep appearing in every reasonable travel-award recommendation you read — it's not that there are only four good partners, it's that flexible points let you pick which four are good this month.
The practical upshot is smaller than the argument suggests. It does not mean cancelling your airline card. It does not mean stopping mile accrual on flights you were flying anyway. It means the following: when you have a choice about where to earn a point — on a purchase that isn't tied to a specific carrier — earn it in a flexible currency. When you have miles piling up in a single-airline program, don't hoard them. Use them before the chart changes, or accept that they're a use-it-or-lose-it asset and price them accordingly. Don't build a plan for a trip in eighteen months around a mile balance you can't move.
A currency you can move is a currency you can protect. A currency you can't move is a currency that only spends where its owner tells it to, at a price its owner sets, on a chart its owner can rewrite on a Tuesday. Airline miles are useful. Flexible points are less sentimental, and less exposed, and — for the trips that are worth planning — almost always the better base to build from.