Alternative web engines are a nice thought, but the reality is that the browser engine cannot be packaged as an "app", it is a new type of application sandboxing environment. Safari (and WebKit and JavaScriptCore) use significant elevated entitlements to be able to do things like control prompts to hardware features like location and NFC, JIT compile code, etc. The reality is that Chrome and Firefox have technical and security limitations which are much harder to overcome than Apple's platform guidelines.