Sorry, but that's one of the stupidest arguments I ever heard (not referring only to you, but also to the parent's parent that brought it up).

Linux is a monolithic kernel, and because kernel parts depend on one another, unfinished changes are often blocking other changes and/or are breaking other parts.

Of course you aren't going to have 1000 developers working on the same line in the same file.

Linux is a product by itself, not a distribution of independent parts. You either have the whole kernel, or you have nothing. And I would die to see another kernel developed by 5 people, achieving the same feature-parity / stability and all that jazz as Linux.