You can develop extensions for the browser on iOS — they just have to use JavaScript inside the page or the share sheet to work. An example of an iOS browser with such extension support would be iCab, though natively in Safari the share sheet is supported too. I commonly load translation apps that way, and once upon a time it was the only way to load LastPass/1Password, etc. And Safari supports content blocker apps natively as well. Not that I don’t support greater choice of defaults and fewer “Reader” apps on iOS, but the situation isn’t actually that dire if you look into it a bit.