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.

I'm also curious about this. Offhand I wouldn't expect it, since even the historical 14-year window is well past where most people buy or pirate a work. Of course, that also implies the time past 14 years isn't about giving creators time to monetize so much as letting a few behemoths protect their brands.