Backwards compatibility is not a given.
Just like CD media wasn't a given for the N64, and DVD players weren't a given for the GCN, I wouldn't hold your breath for it.
Sure, it would be nice, but if its not financially viable, it probably won't be included.
But there are other benefits, such as developers being able to continue developing and selling games based on the GCN architecture, instead of being forced to fork out money for development kits and such, but neither of these factors benefit Nintendo financially.
Sure, it might increase market share, but it won't be a significant increase, and the cost per console will be greater.
Meh. I don't care. I have room for the N5. Either way, I'm happy.