Great observation! I guess the trick is to figure out how to make (1) two very small purchases every month and (2) how to do them automatically.
Does anybody have an idea?
The best way would be to pay a recurring bill like 'net access fees, Netflix membership, or the like. It would be better to carry around the card to buy sticks of gum, single postage stamps, etc. but that's just asking for trouble since you won't remember whether you've made 0, 1, or 2 purchases this month, and you might accidentally buy something expensive with the card.
You would have to make a minimum payment of 4% of the total balance, all of which would reduce the balance at 0%. And then the amount of balance at 15% (or whatever) goes up.
Say you borrowed $10,000 at 0%, and each month you make a $10 and a $35 purchase. You make the minimum payment each month.
Your effective interest rate starts at 0% but goes up pretty smoothly. At the end of year ... your balance is ... and the effective interest rate is:
1 6568 1.23%
2 4465 3.63%
3 3177 7.65%
4 2387 13.57%
Shortly after year 5 begins, your original balance (at 0%) is paid off. So it wasn't "for life", it was for 4+ years, and you wind up with a several-thousand-dollar balance at a high interest rate (unless you can transfer the balance elsewhere).