It seems almost certain that "piracy" would go down. Even if the only change was that people torrenting Firefly in 2019 would be doing so legally, that's that much less piracy.

Then you get the effect that everything from the 20th century becomes available on YouTube, which is likely to cause increased viewership of that content over newer content pirated or otherwise. Which I suspect is the main reason Hollywood is really against shorter copyright terms -- not just losing the revenue from past works but having to compete with them when they're free.