Years after Steve Jobs coined the phrase "Post PC era", people are still debating what this means.
By "Post PC era", I think Apple recognized the focus of innovation, growth, and profits had shifted along with the attention of the masses. Traditional PC software (and hardware) isn't going away, it's just no longer the most dominant thing driving the high tech industry.
Steve Jobs described PCs as trucks compared to the iPad which is more like a car. Most consumers don't need a truck, but that doesn't mean trucks are going away. If you live in the US, almost everything around you was delivered by a truck. Our modern economy is built around moving goods by truck.
To use a simple analogy, virtually all the software for iOS and Android devices was created using PCs (including Macs). The idea that Apple must someday merge their iOS and Mac product lines is like telling Ford Motor Company they'll someday merge their car and truck business because maintaining a separate truck division is too expensive.
For the foreseeable future, this is complete nonsense. The PC and Mac are here to stay.
In the long run, who knows what computers will look like. A good operating system may last 10-20 years. On this scale, Mac OS X is mature while iOS is still young. Interestingly, both are built on top of UNIX whose derivatives have thrived for over 40 years.