Those scary warnings exist for a reason. iOS has s similar installed base to Windows, and it runs on devices permanently connected to the Internet. Guess how many viruses have hit the platform.

There's a reason for that. Getting software onto an iOS device other than downloading it from the App Store is really difficult, and that goes for viruses too. Put that together with the fact that you need to provide apple with authenticated identity to get things into the App Store, and you end up with quite a secure platform

Sure, that security comes with some restrictions. For you that might not be the right choice, but for many people it absolutely is. The Mac provides a platform somewhere in between the two. You can load software from unapproved places, but the system warns you. I think this is a reasonable trade off. Most people do need to be made aware that such actions are risky. They may decide not to load that software after all, and that's a good thing! If you're unsure of the risks involved with loading a piece of software, then you probably shouldn't be loading it.

Finally your point about loading test versions of software on iOS is out of date. This is one of the many major improvements Apple delivered for developers at this years WWDC. The procedure is much more streamlined than previously. At this point it's debatable whether it's easier to manage testers using ad-hoc APKs on android, or using Apple's test deployment system.