I agree it will be interesting to see what develops, given it is possible for anyone with a Mac to install whatever they like, now.
[0] http://stackoverflow.com/questions/4952820/test-ios-app-on-d...
I guess Apple would likely respond by one or both of (a) putting up more roadblocks to getting developer accounts or (b) further restricting what apps can do at a technical level through more sophisticated sandboxes, making the delta in allowed functionality between an app in the App Store and a sideloaded app smaller and smaller.
There's already some significant annoying limitations, in particular short provisioning expiration limits (must reinstall the app every 90 days), and no access to services like push notifications.
You have to rebuild and reinstall the app (with an updated provisioning profile, but Xcode takes care of all that behind the scenes now) to get an expired app working again.
http://www.theregister.co.uk/2015/11/02/apple_clubs_chaos_co...
In addition, people underestimate what you can do from the sandbox (and will be able to do for a long time, unless Apple speeds up its development cycle): https://news.ycombinator.com/item?id=10415288
The only issue with it is that the decision to give everyone a developer certificate for free was to increase Swift adoption. The security implications were not investigated. Apple wasn't aware that they put the final piece of the puzzle in place for something like this to exist.
They can easily reverse the policy.
If this ever happened and got popular, you can be certain that Apple would shoot it down in an upcoming iOS release.
Because that's how Apple works: No freedom permitted on "their" devices.
In fact, Apple even advertises this ability as the most prominent new feature of Xcode 7.[1]
If your response were true, Apple would never have added this ability in the first place.
I can see it now: the "Sideload App Store"