With all due respect, you do know how profit margin is calculated, right? It does not take into account research, engineering, or manufacturing setup. All of these are areas that Apple spends an insane amount of money on and is what makes Apple's products "better". It also does not take into account advertising, or retail costs. Apple doesn't buy better NAND memory than the competition, they research how best to fit it all together. And deciding how best to fit it all together does not show up in profit margin. They use profit to pay for those things. The more profit, the better the product can be, and the better future products can be.
So while I do not doubt that Apple's personal take home is better than a lot of companies (just look at what they have in the bank!) you also can't look at profit margin on a device that has a ton of engineering and research behind it to figure out how much money is left over.
As an extreme example, the "Profit Margin" on a 150 million dollar F-22 is huge, because the raw material and cost to build does not stack up to the price tag. However the "program" (i.e. cost to research and get to the point where they CAN build them) is currently sitting at 66.7 billion.
We never find out what the iPhone or iPad "programs" cost, and how much Apple sunk into those devices before they were even for sale, but they never would have been able to sink the money they did into them if they had a profit margin on the Apple computers similar to that of Dells.
Actually, I understood that they do try to account for some of these things. But it comes down to their strategy for receiving a return on their investment. Sell fewer units for more, or sell more units for less.
Apple's competitors typically seem to go for the latter, so they have to sell more units to cover the investment. This is weird, because this is compounded by the fact that at the same time they typically produce more models which both divides their customers' attention and requires the extra efforts in design, engineering and retooling which all adds up.
In contrast, Apple aims to reach profitability on a product within a certain, much smaller number of sales. And Apple can sustain a higher price per unit because of all the added value (software, ecosystem, quality, support, user satisfaction, etc.).
Creating only one new model per year actually means people have the confidence that their iPhone isn't going to be immediately outdated and obsoleted -- in fact, quite the opposite: it will gain further utility and longevity through software updates. So demand tends to remain quite high even for older models that people perceive to be at the end of a cycle.
But high satisfaction among consumers leads to even more demand for the next model, so there is a virtuous cycle in effect.
Also, by producing one annual model Apple has longer to recoup its investment, when it was already priced to do that fairly quickly anyway. Using the same case design only multiplies this effect, so transitioning from 4 to 4S using essentially the same case allows minimal extra investment that can be used to make real improvements to the phone (vs changing color or throwing the latest component in it as most competitors do). Apple know where to spend its money effectively because they aren't producing loads of models to throw at the wall just to see what sticks. I think they invest far more wisely, accurately and efficiently than anyone else, so they do achieve very high margins time after time.
And of course, they continue to sell the 4, so the margin on that continues to go up and up with every sale. Now remember that the 3GS case design is in about its third or fourth year, so as that continues to sell you realize that anything above actual costs of components is just about pure profit and has been for a couple of years! Who else can make one product cycle last 12 - 18 months, let alone continue to sell "old" models year after year?
So, I don't think, as you imply, that there is a hole for each product program which needs to constantly get filled in. By careful planning, I think Apple frees up a lot of cash that it can continue to use effectively elsewhere. And we know that for its size and all its great products, Apple's R&D budget is actually comparatively quite small. In fact, with hype about new design and capabilities for iPhone 5, with China market coming along, and given that 4S achieved 4 million sales in first weekend, I could imagine iPhone 5 reaching profitability within a month of its launch, regardless how much Apple may have spent on its "program" over the last year or two.
But, yeah, in one sense it's not like Apple chooses to have a certain gross margin that they could lower if they wanted to... it's all in the original bet they made: can we sell X number of phones at X amount in order to reach profitability, and how long will that take? And of course they sell many more than that far quicker than even they imagine. So, in fact, they do lower the price (on the previous model). Then depending on the mix of phones and their capacities, they make a varying amount of margin each quarter.