I can walk someone through the interest math of this strategy in twenty minutes, and the math is almost never the reason it fails. When it fails, it fails in the checking account. The monthly surplus that powers everything erodes, a little at a time, until the line's balance stops falling and starts drifting sideways.
Here's how the creep gets in. The strategy makes you feel liquid. There's suddenly room on the line, and room feels like permission. Dinner out becomes dinners out. The subscription count grows. The vacation gets upgraded because the balance has been dropping all year and we've earned it. Every one of those choices is defensible on its own. Together they eat the surplus, and the surplus is the engine. That's why Find Your Real Monthly Surplus Before You Touch a Line comes first in this whole system. No surplus, no strategy.
The drift is invisible month to month. It only shows up when you compare this January against last January. So track one number: the line's balance on the first of each month, written down somewhere you'll actually see it. Twelve numbers a year. If the trend bends the wrong way two months running, you don't have an interest rate problem, you have a spending problem, and no refinance fixes that.
The fix isn't misery, either. Build the fun into the plan on purpose, a named monthly amount for eating out and trips, so pleasure has a budget line instead of a permission slip. Spending doesn't break the strategy. Unplanned spending does.
And if the balance has already been drifting, don't quit in embarrassment. Recount the surplus, trim what crept, and restart. The math will still be there waiting. Nothing about this strategy expires because you had a bad quarter.