This is the OP, sorry if my tone offended you.
The review board process there is not to the satisfaction of some people. For example, if you create an application and another people want your application but the review board for any reason (real or not) doesn't like it, then you can't distribute it. For example, for some time Apple disallowed applications that duplicated functionality from core apps. So you could not ship a new mail client or a new music player. Yes, those restrictions are gone but for a while they were real. For some months they also restricted the app store for applications developed with their own SDK, if the app was not Obj-C/Cocoa then it could not be accepted. This is also gone but for at least three or four months lots of developers were stranded.
Why am I telling you that? Because this is just to prove that sometimes Apple makes mistakes. Sometimes, their rules are not in the interest of the user. Sometimes, they backpedal and make it right, sometimes they don't. Since you don't have a way to distribute content outside their walled store, you're left without any recourse. The phone is not yours, its theirs.
Now, back to the topic. Browser add-ons also pass through a rigorous review process. Currently, Firefox extensions need to be reviewed and signed before they can be installed on Firefox Stable.
Apple could allow new rendering engines and new javascript engines. Microsoft was sued for similar reasons. Some people are so displeased with Apple WebKit that they are claiming it to be the new IE as can be seen in http://arstechnica.com/information-technology/2015/06/op-ed-...
Having more rendering engines and JS engines would not make iOS less secure. Having add-ons in Browsers would not make iOS less secure since all iOS apps run sandboxed anyway. Also both Chrome and Mozilla have add-on review processess. At Mozilla any open web app or add-on that touches sensitive APIs is reviewed very rigorously. This would not lead to a less secure system but it would probably lead towards more usage of mobile web instead of apps and this goes against Apple strategy and shareholder interest.
The fact they do not allow competition is not good. You should be able to choose what you want to install and from whom.