what exactly are the "system software", which the "customizers" must never ever need to change,
To clarify, I am not saying that maintainers should not modify software. I am saying that if they don't, that making a whole different distribution is overkill and over complicated, and it is much easier for both them and the user to have it as a package instead.
For releases, it would be simply done just like any other package. There's a vast number of packages that already do everything you can imagine, and they have none of the show stopping troubles you speak of.