There's a few parts to it:
- new growth is monoculture, and we're having health issues coming with it (Japan is trying to do something about its pollen problem, but not until two decades from now). It's also more fragile areas, including geologically.
- new growth also means the process that took centuries to build forests is not starting either.
- if the plan is to produce more paper (replace current plastic use with paper use) we can't just keep the current tree farms. It will be expanded and eat up on the legacy forest, as it needs the right niche to efficiently grow.
On the last point, if we were really good at farming new trees, we'd have tree fields not far from manufacturing centers. That's not the case, we're still relying on favorable places where forests used to grow, which doesn't bode well for further mass production in the future.