I'm not just talking about distribution (although you are correct to bring up that restriction, because it's also a kind of wild idea for extension security) --

I'm not just talking about distribution (although you are correct to bring up that restriction, because it's also a kind of wild idea for extension security) -- I'm talking about just how much logic is fully moved outside of Safari. Adguard was (I haven't checked in a while, maybe it still is) one of the more popular Safari content blockers for Mac, and it shipped with an Electron app -- and one that had to actually be running with a tray icon and everything at the same time as the browser. This is not a model that any other browser maker should be trying to copy. I feel like if we're looking at browser adblockers and they're setting up system trays and system-level proxies just so they can do basic stuff like HTML rewriting, then that's not something the browser maker should be proud of. Congrats, people can't spy on your browser's web-traffic anymore, instead they have to MITM their entire computer. It's possible to make something so restrictive that the end result is that people are actually less secure, because they're forced to compromise security at much lower levels in order to get that basic functionality back. Safari's current extension ecosystem is a great example of that.