Actually, I think the iPad has some killer editing apps. Maybe because it's more like using my old dodge and burn techniques in an analog darkroom, but it's a blast to work on photos on it. But I agree it's more of a one-at-a-time deal.
But you gotta consider the direction photography is going for the vast, vast majority of people. Pro and hobbyist photographers are a small market compared to the swarms snapping away with their iPads and iPhones. And as resolution has increased those folks are getting photo collections that rival the size of what a pro might have had in years past. Just type "photo" in the app section of the iTunes Store and see what I mean. Those folks are outgrowing iPhoto, and might need something more like Aperture. Which might not be good news for its future if you're a pro or hobbyist; do you really wanna pay for Instagram like filters?
And consider that most point and shoots and new cameras are gonna come with wifi. That means more syncing with iPads, not less. Remote shooting. Instant uploads to web sites. It's a huge market I expect. Bigger than that for 4K monitors
I'd be curious to see the number of purchases of iPhoto iOS version vs Aperture. And I'd bet lots of folks have at least a Mac and an iOS device. And judging from inquiries like this thread, I'd bet there'd be a big interest in an iPad version; maybe not full on, but at least something that would sync and allow basic operations. LR did take a stab at it, but kinda whiffed IMHO.