What's the point of new champs?

I have a question from the Soraka thread that is a tangent to that discussion.

Why did riot have to break a champ who already existed to "experiment" with a new healing idea for? Isn't that why they should be making new champs. I mean really, what's the point of new champs if you can't test out your radically new ideas on them first?

If riot has a new idea that's radically different, if they want to create champs that don't play like any other in the game, isn't that largely the point of adding new champs? Sure new themes are great, but from a gameplay perspective, aren't new champs the place for testing out new ideas? Isn't this especially the case when the new mechanics are at odds with the old one?

You wanted to try out a new idea. That's fine. That's great. But why not try out something that new and that different on a completely new champ? Why run that kind of risk with an existing champ? I just don't get it.

@morello @vesh @meddler @ghostcrawler