You do know that all 3 share the same basic OS and technologies (kernel, filesystems, basic services, lots of UI and helper code, etc) just in differently tuned versions with a different "facade" on top, right?
Or that that they have different teams on each of these devices for everything that's not shared, teams that don't even communicate with each other...
I love the products. I probably have $15k of Apple hear in my home. But I'm not buying my kid a 3 year old computer to go off to college with at a premium price.
Because the Air and MBP want to phase out in favor of MBPr and Macbook (plain) and they've been waiting on Intel to deliver anything significant to warrant a new product line for ages. (Intel Skylake CPUs in form factors and energy consumption compatible with the MBPr line have been TBD until recently --some still are--, and in limited production yields still. And previous generation didn't even give any performance speed bump of 2 and 3 generations behind MBPr CPU, and in some cases, worse). In the cases where Intel delivered, Apple did too (e.g. Skylake iMac and Macbook).
The Mac Pro has a similar story (and isn't their first priority or biggest seller anyway).
That said, not sure about any "multi-year gaps" regarding the iPad. The iPad Pro 9.7 came out in March, and the iPad Mini 4 less than a year ago, on September.
Working on the watch means that they aren't working on the Mac or the iPad. They are a victim of their own success in that a successful, well thought out, and profitable product like the watch is viewed as a neutral.
They've also flopped on the TV.
That said, their ability to execute is amazing.