If I read the number correctly elsewhere, Apple's annual overhead is over 30 billion. That will prove a significant amount to cover when iPhone sales inevitably decline and little is left but faddish watchbands and puerile television programming—most computer customers since having decamped elsewhere.
Apple is doing just fine. And I guess if Apple's overhead becomes an issue, they will stop buying back shares (some 350B authorized already). I hope you realize that profit is counted
after everything like overhead is taken into account? If Apple
stopped manufacturing a single product of any type today about, what, at least a quarter of their share price would be covered by the cash they have in the bank.
While MS and Google dump 12B here and there down the drain on failed acquisitions like Motorola and Nokia, Apple has similar 12B put on capital expenditures each year, like tooling and CNC machines which it buys and owns, and places on its partners' assembly lines for just Apple products, not small runs of cookie cutter products tweaked for loads of OEMs. Then Apple continue to sell one or two designs for several years (such as the 4S, or the 5S/SE casing), thus rapidly and totally recouping that annual investment and all its R&D and fixed costs. Major reason each iPhone is profitable, and the margin on each actually goes up the longer it sells (instead of throwing 20 new designs at the wall every six months like OEMs do in order to sell a few units as flavor of the month to fickle customers).
Oh, iPhone sales will decline at some point. Just as the iPod did. That's why they now have the iPad and Watch. Both are big businesses on their own. The iPad did take a big dip for a while, but is coming back up, and is arguably still the only successful tablet on the market.
The Watch is more successful than people realize, Apple just doesn't share numbers. Analyzing the revenue and other things that Apple does say about that side of the business, Asymco.com puts the Watch revenue as already surpassing peak iPod revenue, and rising (and that was Apple's bread and butter of the day, before the iPhone, which Apple was happy to let cannibalize sales of its own iPod). It is notable that some insurance companies are supplying/mandating Apple Watches for their customers. The health side of Apple Watch is going to be big.
Then there is the Apple Car

. You can read about this on Asymco, too. If that becomes a thing, it will be big... the thing is, Apple is probably waiting patiently as it works on a way to revolutionize the manufacture of cars into a modular process. That's the thing about Disruption -- it modularizes and integrates different parts of the product or process in new ways, to create a new basis of competition. Yes, Apple's complete product is most often "integrated" (it does both the hardware and software, for example). But it's beating the competition in many ways (such as performance) because the competition relies, for example, on integrated, off-the-shelf SOC's, while Apple has "modularized" the SOC on its own, exclusively for its own use.
So, Despite Tesla saying it wants to revolutionize the car industry, it really hasn't: it is building electric cars in traditional ways -- and having lots of teething problems in the process. Their production on some lines has all but halted as they try to increase capacity to just normal car manufacturing efficiencies and levels. They were producing/selling, what, some 50-80K per year. They want to what, double or triple that? It is really going to take an order of magnitude more to affect or "transform" the car industry (with a new approach, such as new methods of manufacture with true modularization embedded in the process).
This analysis is related to the idea that Jobs targeted 1% of the smartphone industry. One percent of the global car market of 80M or so units is 800k cars! Asymco surmises that Apple should bring its supply chain experience (where it purchases the equipment for assembly lines for its partners, and dictates the whole process, etc.) into play; and that if/when Apple finally makes a move into cars, it would be with the idea of not making a few premium products on the lines of what we have now with Tesla, etc.-- but perhaps a lot of cars of a completely different nature, with a different basis of competition as something actually disruptive (Tesla's approach in the industry is actually "sustaining", not disruptive by technical definitions).