These alternatives wouldn't exist (or be so widely used) if the web provided the benefits that these tools give.
For example, look at how many people switched from CoffeeScript to ES6/7 once these languages started providing most of (and not all of) the benefits that CoffeeScript provides.
In all honesty, I don't think sourcemaps are the answer to the black-box problem.
If I'm right, we won't see wasm overused such that the web becomes a sea of monolithic and opaque apps (as too many mobile OS homescreens are). Per the FAQ, wasm is also for hot kernel code and reusable libraries helping mostly-JS apps.
It's up to web devs to keep innovating ahead of browsers, to keep pushing for better HTML, CSS, JS, and beyond. One way to do this might be via wasm modules that extend the web. The https://extensiblewebmanifesto.org/ project continues.
And I'm not sure if that's gonna be true.
For example, I'm sitting here, arguing for an open and readable web, and yet I can't wait for a rust-to-asm.js workflow to get stable enough so that I can move the perf-critical parts of my app to rust (you know, the part that could be learned from and hacked on by another developer).
WebDWARF then would be to WebASM, as DWARF is to ELF files.
Anyway, I think I should explain my thoughts better instead of only pointing out a problem. I wrote this comment: https://news.ycombinator.com/item?id=9743859
For example I may chose to write my code in Rust and that imposes a burden on you (assuming you know only Javascript), but it does not curtail your freedom. Not as convenient as before for you since you now need to learn Rust. I think the freedom has spread to the person who is creating the content (code). The general consumer does not care, but the tinkerer/hacker now needs to put in more effort to parse the creation.
If it's a one-way Rust source -> wasm transformation, even a Rust user might have a hard time following what wasm spits out.