You make excellent points. For arguments sake, let's break this down.
Say customers are buying less pad because it's a year behind (with a lower price). They can 'see' what's coming in the phone, decide it's worth the wait (like say 3d touch or like I did with the stylus) and still buy but delayed.
So Apple comes to their senses and puts pad on parity with Phone. Now pad has all the goodies when the phone does and there's no delays. But pad also costs $100-200 more. There's a big initial rush because now we've got two years ahead instead of one year ahead on, the next generation. But the year after that and the year after that, the steps are smaller again. And the price doesn't go back down. This would be like buying an iPad mini 4 in 2013 for $600.
The question is, are pad sales slowing because pads are a step behind or are pad sales slowing because pad sales are naturally slow. Put another way, are people more willing to upgrade their phones every year because they wear them and not pads because they don't? I only upgrade either every 2 to 3 years so I'm not the best example.
IMO, the market is mixed, those that can do what they want with one device (iPhone) and don't need the iPad. Those that want/need both. This is because the iPad is not really a true mobile device like the iPhone is. Even with phone in the mini, it's still not the same type of mobile device because of the size.
When you go somewhere, you grab something that's not cumbersome. The largest iPhone is on the edge of cumbersome whereas the mini w/phone service is close to the edge of being too large.
How many of your pockets can fit a mini? This was one of the big points of the Nexus 7.2, it could fit many more pockets.
Apple has to realize this, so the market for mobile business users is much smaller than regular users.
Another issue here is what's next. Apple knows that it's more important that they be the last phone/pad you buy when the devices have all the stuff people want.
Ask yourself, what would the iPhone 7 have to have in order for you to dump the 6s+? How much better would the cameras need to be?
How much faster would the CPU need to be?
How much larger would the screen need to be without being too large?
How much more battery life, how much thinner?
At some point, the upgrades slow and that's when Apple's HUGE cash flow slows.
In business, the holy grail is high margin / reoccurring revenue, Apple doesn't have that in their product line for much longer. Stocks are based on future sales/profits, how's that look for Apple?
Remember the computer sales became razor thin margins after a while, even top end servers had limited sales.
I'm using the same computer for about the last 5 years with no plan to upgrade, no compelling reason to, it was a rocket ship when I bought it and it's still working great for my needs.
If Apple is smart, they'll focus on high end/high volume enterprise apps. I can't see them replacing the iPhone level sales with TV or watches.
