On one hand, sure, the entire point of a programming language is to make complex ideas able to be expressed in simpler abstractions. On the other hand, we can damn well try.
IMO the very minimum requirement should be that you've demonstrated effort to reduce unnecessary complexity of the problem. Sure, some problems are complex enough that there might not exist an obvious solution, yet usually after a while once you're familiar with some topic the existing solutions do start to appear obvious. If they're not I'd argue we're doing something very very wrong
I think it's also worth distinguishing _problem complexity_ and _solution complexity_. The problem might be really really hard (and it very obviously is in the case of adding multi-threading to JavaScript). But it does not mean that the solution has to be hard to understand. It doesn't mean that any average PHP developer (I can say that, I started with PHP) should be able to verify the correctness of the patch, but for a person who is well familiar with the area there shouldn't exist areas they can't understand.
Look at the description of your own Fil-C: it focuses on clarity of explanation of how it works, and it actually does make sense (and, hopefully, works well enough too). Compare that with the pull request sent here. I'll wait
I think LLVM is a perfect example of what happens when it's too complicated: it's slow, it's bug-ridden when you stray away from the beaten path (e.g. Rust hits bugs in LLVM like this one https://www.reddit.com/r/rust/comments/l4roqk/a_fix_for_the_... ), and it's really hard to use and understand.
It's obviously not useless because of that, but it's a great example of what happens when you cannot fully control the implementation complexity