Does any paper at this point come from old growth? I'd figure its new growth treefarms that produce paper. At which point, you can't exactly flip a light switch and expect a complex forest ecosystem that took a millenia to establish to restore itself on human timescales. And thats only if the existing landowners wanted to just say "we are content to not make any money ever again with this land"
- 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.
Seems like the manufacturing centers need to be sited on rivers with rights to pull from and with access to sufficient available power, and a way to distribute this product. Are all forests in such condition? Probably not. I've seen some of these forests, they are often in the middle of nowhere connected by dirt roads built by the logging companies. This is marginal land, sometimes leased by the US forest service. Prime land near rivers and power sources isn't going to be squandered when you can get a permit to farm on marginal land or buy it for cheap if its a private holding, just like how you don't see grazing near feedlots. Grazing needs more land, can be done on marginal land, and you can just ship cattle to feedlots elsewhere near meatpacking plants. As a result we graze cattle in places where you sometimes can't even drag a plow, and have feedlots near chicago where cattle can be processed in vast quantity and then distributed.
I will agree that monoculture is no good and neither is taking over virgin forest, but this isn't exactly the only industry on this planet that behaves like this either. Rather than trade paper for plastic we can instead consider how we protect virgin areas from any sort of development. Monoculture is also a risk for industry in the form of crop loss from disease, so I wouldn't be surprised if this begins to change in the future and we see more polycultures farmed, not just in trees either. So if there is a plastic ban favoring domestic paper products instead, as well as sufficient protections in place for existing virgin forest, you can probably expect the value of a tree farm to go up to the point where people who might be using their land for one thing being more inclined to farm trees on this land.
Paper can be made from reeds and cereal straws. However harvesting reeds will destroy bird habitat and straws are also used to feed livestock during winter.
It's complicated.