Most performance aware modelers would consider an engine worn-out, when it is down 11% in maximum HP output...
...Which is the condition of a TT Pro is, when it is down 500 RPM, from new...

It is possible that no further wear will occur, since there is nothing to wear any parts down in a loose tapered-bore engine.

Dust and sand, maybe...

If the engine becomes harder to hand-start, as it will be when it becomes loose; it to me needs an overhaul.

In a ringed engine, the force applied by compression and combustion to the ring, on the sleeve, will continue to wear both of them down.

And Brian is correct. The piston will most likely be reusable.

A steel sleeve, or a nickel plated one, will be worn by the ring and will eventually need replacement.

A chromium plated sleeve, like in some older K&B, ST and even an OS, or two, may survive many more ring replacement cycles.