There are a couple of problems with this proposal. First of all, it would prevent you from using webviews to implement any kind of browser. Which in the case of iOS means it would impossible to make an alternative browser to safari, because you aren't allowed to use an alternative browser engine.

Secondly, a lot of native apps rely on using websites controlled by the same entity that have x-frame-options set, and those would break if that suddenly caused it not to open.