-Reduces explanatory power (if Moby-Dick and 50 Shades are the same, why has one been lauded for 100 years, minted the careers of countless academics, etc.?)
-Makes the world less interesting
I don't know, but those sounds like some plausible answers.
On the second point, I vehemently disagree, and I'm curious how you think a larger, more complicated world can be argued to be less interesting? Does the fact that your hallucinations and delusions are vacuous, and that they don't sit alone in a dark void, paralyze you with fear?
But take Hamlet, for example. You can say that it's about revenge destroying the one who takes it. Or you could say it's about failing to seize the moment ultimately making you pay a much higher price. Or you could say it's about insanity, and whether sane people are insane and insane people are sane. And none of those is wrong!
You could even say it's about how tedious it is to have to listen to pompous old windbags, and you'd be at least partially right.
But if you said it was about body thetans, then no, you are objectively wrong. There is no honest reading of Hamlet that reaches that conclusion.
"Thickness" is giving authors credit for things they didn't write, but they did leave breadcrumbs for. That's different from "they didn't write it at all", and it's also different from "I didn't look for breadcrumbs, so I didn't see them".
Here's an example: The song "Welcome to the Future", by Brad Paisley, the third verse says:
I had a friend in school
Running back on the football team.
They burned a cross in his front yard
For asking out the homecoming queen.
I thought about him today
And everybody who'd seen what he'd seen
From a woman on a bus
To a man with a dream.