Indeed they do. I can think much of the NPM ecosystem which assumes (incorrectly) that packages are unique, stable, and good. That whole situation a while back showed that not to be the case.

I'd be OK with grabbing code elsewhere, as long as I could guarantee that it would never change. Primarily, immutability. I'm thinking of something like an IPFS repo which is "elsewhere", but still very much crawl-able, scannable for bad stuff (aside turing issues...), and can be shown to reproduce stuff if it is broken.

Also, using an immutable self-certifying system would also solve the second point, regarding the single MITM. The trust would be with the file/package, and not some ephemeral cert (whos trust is brokered from above).