"Trees grow back"

Yep...but understanding what that means is more than relevant. The old trees (100 years +) with LONG fibers, are habitat and important parts of the ecosystem....tree farms with short fibers rely on different types of faster growing timber.
The problem is similar to tuna fishing-- we are using the older, bigger, more desirable ones faster than they can be replenished. So- we are merely putting off 'running out' to a later date. Someone...whether 20 years or 100 years from now... will have to wipe with something else.

Any solution? Sure....reduce the population to...oh, 50 million, instead of 300 million. You'll have both tuna & TP to suit.