The offset every dollar you don't spend
Institutions never let reserves sit idle against a liability — they offset the cash so every dollar works at the liability's own rate. An all-in-one or HELOC mortgage applies the same mechanic to your home loan: your paycheck offsets the balance the instant it lands, and interest is charged only on what's left after your cash flow does its work. The question this tool answers honestly — how much of the famous "pay it off in ten years" is the offset, and how much is just paying more.
The drama in every all-in-one pitch — thirty years collapsed to ten — is almost entirely the copper bar: the discipline of pointing your whole surplus at the principal. You can do that today, for free, on the loan you already have. No new product, no variable rate, no closing costs.
The all-in-one's own contribution is the thin offset bar: the interest you save by letting deposited income and a standing cash buffer sit against the balance instead of earning nothing in checking. That edge is real, but it is small, and a rate premium of even one point can erase it — which is exactly what the default settings show. The fiduciary question is never "does this pay my house off faster than doing nothing." It is "does the offset clear the premium I pay for it." Slide the all-in-one rate toward your mortgage rate and watch the moment it does.
Model assumptions, all adjustable above: surplus is routed identically across the extra-payment and all-in-one cases, so the only difference is the interest engine; the parked buffer offsets in the all-in-one and earns nothing in the conventional cases; rates are held constant (a real HELOC floats). This is a teaching instrument, not financial advice.