I know, but I am just questioning whether any license is needed for open source to prosper.

I am positing that if licenses didn't exist, and anyone could do anything they want with any bit of code they see, open source would still prosper.

That just isn't how things work. I believe you are making the wrong assumption that the same amount of open source work would exist. If that were the case, then yes, it wouldn't matter that much. But, a lot of contributions to open source wouldn't have existed if it wasn't for the licenses. That's the examples you've been given.

There might be some contributions that wouldn't have happened, but there also might have been others that did happen. My hypothetical was in a world with no copyright or license at all.... so all proprietary code would be copyable, too

Proprietary code would by definition not be copyable, because it would be... proprietary. It's the opposite to open source. The thing that incentivises open sourcing proprietary source code is exactly things like licenses... Your imagined scenario is just "no license for open source, and free reign for closed source". It makes no sense to me.

If no code was copyrightable nor licensable, people could reverse engineer any code they get access to. It would be hard to keep code completely proprietary. You would not be able to distribute your code at all.

I am not saying this is the way we want to go, I am just curious about the thought experiment.

Ok, if it's for a thought experiment then I'll play along. Reverse engineering source code is not as easy as you make it out to be. Not only that, but reverse engineering in the sense of black box is not a violation of most existing licenses. And, if we're talking about decompiling code into something useful, that too is a tall order.