Except if we want to make a proper analog to the Mac, they don't actually do this. The Mac updates when it can, with the tech it can. There's yearly spec bumps when Intel makes a new chip available, and they tend to tie in new hardware tweaks as these new chips come in. But part of this is that Apple can't force Intel to stick to a release schedule, so they can't make the schedule all that rigid even if they wanted to. But Apple does want folks to be able to buy the latest hardware when they do make a choice. Updating the Air every 2 years runs counter to that, considering it is one of their more affordable products.
If the iPad is really moving to a Mac sort of model, it is not something that updates once every two years. But what it is likely moving towards is a model where they don't need a media circus for every refresh, nor do they need to push it for holiday if a spring release makes more sense.
I'm not saying you are wrong, but I'm saying your reasoning holds no real weight behind it. Because what you point to as an example isn't actually doing anything like what you claim.
