Ten-year copyright would mean Marvel’s “phase two” movies competing with off-brand Iron Man sequels… and that’s ignoring how the character and all his good stories were public-domain for decades.

Ten-year copyright would mean the last Harry Potter book and half of the movies competing against commercial fanfiction. Just a flood of contradictory cash grabs which admittedly would not fund an obvious bigot.

Ten-year copyright would mean Game Of Thrones didn’t owe GRRM anything, if they skipped or delayed season eight, which frankly would have been an improvement. Admittedly it also means there’s been five years where anyone could publish their own damn version of The Winds Of Winter, and that might be the only way it ever comes out.

You say these as if they would be bad things.

In most cases, people will still prefer the 'official' sequels and such from the original creator. Because if you liked the original, then you'll want more from the same creator, and maybe not so much from other, unknown creators.

To take one example, Copious fanfic of the Harry Potter books does exist, and it even existed well before the final books were released, much earlier than 10 years. Some of it was (I assume) actually pretty well-written and creative. And unlike the official books, it was available for free. And yet, readers of the franchise, overwhelmingly, preferred to pay to read the official sequels, rather than read free fanfic of it.

Could this result in slightly less profits for content creators of series works? Sure, maybe. But there's also the chance that these off-brand sequels could be good. Perhaps better than official sequels. Perhaps even better than the original. Having more choices and more options out there can only be good for the consumer. And it might even be good for the content creators as well -- it will motivate them to keep quality high during later parts of the series, since they can't depend on IP protection and being able to ride on the coattails of previous success. Since disappointing sequels are pervasive in our media landscape, perhaps that will be a very good thing indeed.

You're already stumbling into that conclusion with:

would mean Game Of Thrones didn’t owe GRRM anything, if they skipped or delayed season eight, which frankly would have been an improvement. Admittedly it also means there’s been five years where anyone could publish their own damn version of The Winds Of Winter, and that might be the only way it ever comes out.

But the biggest part is just giving consumers more options and more art. Just think of all the great art we might be missing out on, art that would have been made if copyright law didn't prevent it.