FWIW you can install alternative browsers with alternative search engines in iOS as well. I regularly use a Tor browser, for instance. The only thing that's fixed is the Webkit engine, but that's immaterial to this discussion.
It's not quite immaterial. You can only use an alternate search engine on iOS if you install a slower browser due to Apple withholding the JIT capabilities of Mobile Safari from 3rd parties, thus artificially propping up Mobile Safari as the fastest iOS browser. Although Chrome on Android limits you to only the 3 bundled search engines, alternative browsers expose additional choices without being artificially limited by Google.
Now the web view runs in a separate process with a separate sandbox, so it can run the same JIT of Safari without security concerns (mainly, writable memory with execution permissions).
