Browsers are very complex and expensive to keep cutting edge.
I think it would be an interesting, more private, and safer online world if browsers only displayed HTML and a minimum of JS rather than being entire operating systems, and anything like media playing and high interactivity occurred in native programs.
https://git.savannah.gnu.org/cgit/gnuzilla.git/tree/makeicec...
Pale Moon did this a while ago and is still around, although in need of donations as many other volunteer-driven projects. https://www.palemoon.org/
People assume Pocket offered Mozilla a lot of money, as there's really no other reason Pocket shouldn't be an extension.
People are naturally nervous when browsers start deploying third-party code in exchange for cash. Especially when auto-update mechanisms are used.
Some would see that as an important line to cross - will they be able to resist the much larger sums they'll be offered to "help users see more relevant ads" and "help users keep safe from viruses"?
That's how I've always seen it anyway. I didn't know there was controversy around it, but I can understand how people would be sensitive to seeing another company's logo in the URL bar.
There is no good reason for it to be "core functionality" in a browser, other than the fact that Moz://a got a fistfull of dollars and foisted it upon us.
EDIT: For example, what would be a good use of core functionality, is if Moz://a included IPFS resolution and native support. That is a protocol level support, and thus would need native support rather than plugins. Plugins do indeed work, but require significant workarounds. For example, native could be ipfs://hash whereas the plugins need to specify http://127.0.0.1:8080/ipfs/hash