You don't need to root an Android device to load applications from outside the Play store. The security warning is pretty accurate and useful for most people but if you want to load apps from outside the store then you aren't most people so it shouldn't matter.

There's a reason alternatives to iOS and Android haven't picked up, some of those reasons are because for those that care to side load apps they can already do it on Android (or can go further and replace their OS) so not much incentive for a third mobile OS. If there was a strong enough need for an alternative then that would exist already.