When a good deal comes along, especially if it doesn't have a designated expiration date, folks often talk about whether the deal is "sustainable" or not. The general idea is that if a deal is "too good to last," then it won't.
Of course, there are lots of ways a deal can end. If it's ended retroactively, those who jumped on it quickly will find they've wasted their time, or worse. If it's ended going forward, the prospective benefits of a credit card application may be cut short, or someone can be left with a garage full of merchandise they have to return or resell at a loss.
I think there are three slightly different issues related to sustainability that guide how I think about how long a deal is likely to last: value, cost, and risk.
High-value deals aren't particularly vulnerable
For $75 per year, anyone can carry a Hilton HHonors Surpass American Express and earn 6 HHonors points per dollar spent at grocery stores. Applied to certain high-value redemptions, like a 5-night stay at a premier property like the Conrad Maldives Rangali Island, that might work out to a roughly 14% return on your grocery store spend (for a sample reservation from December 31, 2017, to January 5, 2018).
That's a great value. And since it costs American Express just a fraction of the value the cardholder receives, it's not particularly vulnerable. After all, American Express doesn't care where you redeem your Hilton points, they care what they pay for them, and they pay much less for 6 HHonors points than they earn on your grocery store swipe fees.
Likewise, the US Bank Flexperks Travel Rewards card offers "up to" 4 cents per dollar spent at grocery stores, but it's not like you get a check every month. Instead, you have to save up enough points to redeem for a flight you're planning to book. Then you have to hope the fare is close to the top of a redemption band. It could take the average customer years to save up enough points to redeem for a single flight, during which time they've paid multiple annual fees and they haven't cost US Bank a dime — in fact, they've been a profit center. That's a high-value deal to the travel hacker that has nonetheless proven extremely resilient over time.
High-cost deals are vulnerable in the medium-term
Compare that to the original "old" Blue Cash card from American Express, which offered 5% cash back at grocery stores and drug stores. Admittedly, cash back accrued with a 2-month lag time, but you could earn unlimited cash back far in excess of any swipe fees on a card, and with no annual fee. The "old" Blue Cash card was a loss center, and American Express noticed. They shut down some heavy hitters and transitioned the remaining cardholders to the product they continue to offer, which limits bonused earning to $50,000 of spend per calendar year.
Banks and other merchants have proven willing, but not particularly skillful, at shutting down opportunities like this. When you find an opportunity that moves cash directly to you from a bank or merchant, it's a good bet the opportunity will be closed within 6-18 months.
High-risk deals are extremely vulnerable
In my experience, banks don't seem to mind customers who grind away at them day in and day out. The reason isn't any secret: acquiring a single customer who runs up credit card balances they're unable to pay off covers the costs of many people happily earning 1-2% per month. A fisherman doesn't get at angry at all the fish he doesn't catch; he knows the more fish there are, the more likely he is to land a big one.
But unprofitable behavior is different from risky behavior. Spending a multiple of your credit limit each month isn't likely to get you shut down because it's unprofitable — lots of things we do are unprofitable in the short term. Spending multiple times your credit limit each month is likely to get you shut down because it's risky — if you look like you're struggling to juggle your credit limits across multiple cards, it creates the (not unreasonable!) fear that a particular bank might be the one left holding the bag.
That's not to say risky deals aren't worth pursuing. They're often very worth pursuing! But the riskier your behavior looks to the other participants in a deal, the more rapidly it's likely to be shut down — even if it's no more or less profitable than a high-value deal that's been available for years.