These ideas don't scale though - a 404 handles everything, not just textual content pages (meaning like images, css, js, etc. - the underlying plumbing for most

These ideas don't scale though - a 404 handles everything, not just textual content pages (meaning like images, css, js, etc. - the underlying plumbing for most sites). Nobody is going to sit there and write 404/410 content for moved assets one by one when they rebrand their site, that's madness. In my travels, most folks who actually care about this (SEO links) build an extensive alias/redirect map for the old/previous core URLs they want to remain functional as part of their rebrand, when the new site goes live the redirects are dropped in as well. This is especially true if they've ever published the URLs on physical media (mailed postcards, e.g.).