Iwata himself has said that they weren't replacements. I don't remember which interviews but I know there have been at least a couple of articles talking about it...
You severely misread that quote if that's what you got from it. He said it wasn't a "simple" replacement, not that it wasn't a replacement at all. As in, it's not just an iterative upgrade. That doesn't mean it's not succeeding them. It means it's not something direct like the transition between Wii to Wii U, but something more complicated.
In other words, the unified platform with a shared library across two or more systems. It's not replacing one or the other - it's replacing both. Not a simple replacement. More complicated.
Well, this is new.