Apple famously cannibalizes their existing products deliberately, so I don't think that's what's holding them back. It's also strongly possible that Apple's eventual product will require an iPhone to function, so cannibalization might not even be an issue.

Apple also famously tries not to release products first, waiting until they can deliver something that fits well into their ethos. How well they succeed at that is an open question, and whether it's a wise approach is an open question, but it's their approach.

I'm not sure Apple can deliver another home run, but so far they've delivered more than anyone else I can think of, so I wouldn't bet against them either.