Some tasks can be crowdsourced, and others cannot.

The tasks that can be effectively crowdsourced are precisely those that can be parallelized. Wikipedia works because someone editing an article can be ignorant of 99.9% of the other articles on Wikipedia and still do a good job. Attempts to crowdsource novels have failed, because editing any part of a novel requires a deep understanding of the whole.

Is crowdsourced fiction possible?

Yes.

The SCP Foundation works because there is no canon. There is a tone, themes, recurring characters. But, fundamentally, the SCP is a collection of loosely-associated articles, each of which stands alone.

The Myst fandom has the Guidelines for Official Story/Age Creation, sometimes called RAWA's Rules of Writing. The core of these rules is: Don't create contradictions.

There's some nuance to this. It includes not contradicting canon, but there's more to it than that. Fan writers are expected to avoid contradicting elements of canon that have not yet been revealed. In other words, writers are supposed to avoid writing anything that might be going to be contradicted later.

This is surprisingly possible.

The trick to writing in a massively shared world is to keep the scope of each story small. If you write a story about Martians landing in Missouri, this is likely to affect lots of people, so any other stories set in the same world will have to address the Martian issue. By contrast, if you write a high school social drama, people in the next town over -- or even the next classroom over -- might very well never be affected at all.

The key thing about small-scope stories is that lots of them can fit in a single continuity. Once you've agreed on a setting -- and if collaborative worldbuilding is too hard, you can always just write earthfic -- you can write stories in the same continuity, without contradictions or plot holes, even if none of the authors ever read each other's stories.

In other words, it's parallelizable.

(And, of course, you can cross-link stories if you want. But then you only have to worry about compatibility with the stories you're explicitly directly linking with, not all of the stories that have been written in the setting ever.)

Of course, something like this is only possible with a Free Culture license. But it's possible.

And, I think, it's worth doing.