"No one will consider it authentic or canon." Citation very much needed. When there's demand for something, people will take what's offered. I would be fine with some sort of easy licensing scheme that would allow others to write stories, etc., in someone else's universe / with their characters... but they should see a taste. Especially when we're talking about mid-tier or lower-tier creators who are probably depending heavily on that large body of work to keep the lights on.
That's not true. It might make it harder to make as much money, but there's nothing that says the author can't still profit from selling something after it's in the public domain. I've paid for works in the public domain multiple times, sometimes directly to the author of the work.
Creators have a massive advantage when selling their own stuff. They can include things like signed copies and extras that no one else ever could. Fans want to support the creators of the things they love because they want more of it.
We've already been robbed of all the amazing creative works that might have been but never were because of excessive copyright restrictions. It's hurting artists and it's hurting our culture.
We want copyright to give authors a chance to profit for a limited time, but it we need it to be much less excessive and restrictive so that new works and new artists can thrive.