Apple has an iPad problem. I never said it has to be as big as iPhone. Tablet sales are stagnating for several reasons. One of them is people do not feel the need to upgrade them because new features are not needed. I have seen this both with myself and family.
Also, the tablet has taken a huge hit in primary education. It started out as almost the holy grail, but as schools tried to use it people have realized it is not ready for productivity. It can be used for certain tasks, but in general a laptop is better. The cost of an iPad is still high compared to Chrombooks. I work in education and see this within my school district. Let's not forgot the disaster of LA schools. If Apple wants to get serious, they need to make educational software and services and do it soon.
As phones increase in size, people are also less in need of tablets. I personally am seeing this in the family also, with someone dumping the iPad mini/iPhone 5s for just the iPhone 6+.
I love the iPad; its a great device. But the integration between iOS and OSX is no where near as good as Apple leads people to believe, especially for serious work.
I DO NOT want a merged OS. If they create a 12" version that runs iOS when on its own, and boot into OSX when attacked to a keyboard/trackpad, that could be cool. A merger will only lead to a water-ed down OSX rather than a beefed up iOS. We have seen how Apple merges those two with iWork and such, and the OSX features always get the short end of the stick.