There's a reason internet standards progress so slow on iOS. It's because Apple disallows any competition. If they allowed real Chrome and Firefox they'd be forced to keep up. Users would have a choice. Chrome has proven to to be around 10x as secure as Safari on macOS based on exploit report. There's every reason to believe it would be the same on iOS. I'd prefer to be able to choose the more secure browser with more modern features. that's would be a pro user feature to me.
As other comments mention, all a mandatory browser leads to is "you must use IE6 to visit this site" which was a very terrible world to live in. It isn't happening now simply because there are more !iOS devices than iOS devices.
It's not that I don't want to use a 3rd party web browser. It's that I don't want a 3rd party web browser to ever become popular enough to give that browser's vendor the ability to influence Apple's ability to make technical decisions about their platform. Being beholden to 3rd party software vendors was basically the story of OS X for years. I don't think it was good for Apple.
> As other comments mention, all a mandatory browser leads to is "you must use IE6 to visit this site"
On the contrary, as long as Apple is Safari only and Android is mostly Chrome, there is no way that this can happen.