You can install other maps/browsers/assistants on iOS too, and other apps can interface with them if they want to. For example Chrome will automatically open YouTube links in the YouTube app. I agree that iOS is more restricted in that it doesn't let the user specify the default browser etc. for links from other apps, however.
Alternative browsers on iOS are forced to use webkit engine. Also, I don't think you can install alternative dialer/SMS apps, can you?