I'm concerned about the fact that the iPhone makes up nearly 70% of Apple's revenue. If somebody makes a better phone, Apple crashes. I know that's unlikely given that people have been trying quite hard even since the first iPhone and haven't managed it yet, but still, talk about putting all your eggs in one basket.
Could it happen? Sure.
Would it happen? Probably not very likely in the near future. I don't think this is something to worry about for now.