This is not necessarily a good thing. This leads to traps where someone might have came up with something, put it on the internet, you never read it but you make something similar and they accuse you of plagiarism.
So what? There is no legal liability attached to plagiarism; copyright infringement is the only legal issue. Plagiarism is merely a moral, ethical issue that share elements of copyright infringement. And to your example, copyright infringement has a defense of independent creation. Among other things, the author would have to show access to the "infringed" work.
The same thing will happen, just with a large publisher with a huge sum of cash. The copyright never falls into the public domain, the artists get screwed and orphan works (as in works that are never released but still have copyright on them) still happen.
Of all the works in the world, works owned by a "large publisher with a huge sum of cash" is likely the minority of all copyrights.
Besides, the point of the above plan is not to deprive people of their copyrights. It's a plan to balance to trade-offs: continued protection for those that see a continued economic value and release to the public so that others can exploit them.
Again, how is this a benefit? Look at Shakespeare's works, most of them were adapted from works that would still be under copyright if your system had been in place when he was alive. It is a natural part of art to borrow and adapt.
Copyrights don't protect the ideas of stories. They protect the actual physical work themselves. Ultimately, this comes back around to conflating plagiarism with copyright infringement.
Nevertheless, I'm not about to argue the merits of applying modern copyright law to a different era.
if an artist doesn't think something is worthwhile they won't publish it.
You don't know many artists, do you?
Also, there is a great deal of copyrighted work that gets published once and is essentially abandoned. Look at the software companies whose assets essentially go abandoned, or whose products and solutions have no current use because of hardware and platform changes.
If it isn't published it isn't copyrighted, even after their deaths if someone takes it and publishes it they still have the copyright from when they publish it.
So? I fail to see your point. If it isn't published no one gets the benefit of it anyway and if it is ultimately published then the same rules would apply, no harm no foul. The only questions might be over ownership of the copyright.