I agree you should be able to set a default app (to a degree). But you can just as easy use whatever messaging app you want to use with no issue.I (we) should be entitled to set a different default messaging app on iOS, though.
For example, i almost never use the iPhone camera for selfies. I literally use the snapchat camera. Not that hard.
If you want default everything where everything is not working as one central unit, go to android.