Steve Ballmer’s surprise announcementthat he will be resigning as Microsoft’sC.E.O. has set of f a huge f lood ofcommentary. Being neither a tech geek nora management guru, I can’t add much onthose fronts. I do, however, think I know abit about economics, and I also read a lot ofhistory. So the Ballmer announcement hasme thinking about network externalitiesand Ibn Khaldun. And thinking about thesethings, I’d argue, can help ensure that wedraw the right lessons from this particularcorporate upheaval.

First, about network exter nalities:Consider the state of the computer industrycirca 2000, when Microsoft’s share pricehit its peak and the company seemedutterly dominant. Remember the T-shirtsdepicting Bill Gates as a Borg (part of thehive mind from “Star Trek”), with thelegend, “Resistance is futile. Prepare to beassimilated”? Remember when Microsoftwas at the center of concerns about antitrustenforcement?The odd thing was that nobody seemed tolike Microsoft’s products. By all accounts,Apple computers were better than PCsusing Windows as their operating system.

Yet the vast majority of desktop and laptopcomputers ran Windows. Why?The answer, basically, is that everyoneused Windows because everyone usedWindows. If you had a Windows PC andwanted help, you could ask the guy in thenext cubicle, or the tech people downstairs,and have a very good chance of getting theanswer you needed. Software was designedto run on PCs; peripheral devices weredesigned to work with PCs.

That’s network externalities in action, andit made Microsoft a monopolist.

The story of how that state of affairs aroseis tangled, but I don’t think it’s too unfairto say that Apple mistakenly believed thatordinary buyers would value its superiorquality as much as its own people did. So itcharged premium prices, and by the time itrealized how many people were choosingcheaper machines that weren’t insanely greatbut did the job, Microsoft’s dominance waslocked in.

Now, any such discussion brings out theApple faithful, who insist that anythingWindows can do Apple can do better and thatonly idiots buy PCs. They may be right. But itdoesn’t matter, because there are many suchidiots, myself included. And Windows stilldominates the personal computer market.

The trouble for Microsoft came withthe rise of new devices whose importanceit famously failed to grasp. “There’s nochance,” declared Mr. Ballmer in 2007, “thatthe iPhone is going to get any significantmarket share.”How could Microsoft have been so blind?Here’s where Ibn Khaldun comes in. Hewas a 14th-century Islamic philosopherwho basically invented what we would nowcall the social sciences. And one insight hehad, based on the history of his native NorthAfrica, was that there was a rhythm to the riseand fall of dynasties.

Desert tribesmen, he argued, alwayshave more courage and social cohesionthan settled, civilized folk, so every oncein a while they will sweep in and conquerlands whose rulers have become corruptand complacent. They create a new dynasty— and, over time, become corrupt andcomplacent themselves, ready to be overrunby a new set of barbarians.

I don’t think it’s much of a stretch to applythis story to Microsoft, a company that did sowell with its operating-system monopoly thatit lost focus, while Apple — still wanderingin the wilderness after all those years —was alert to new opportunities. And so thebarbarians swept in from the desert.

Sometimes, by the way, barbarians areinvited in by a domestic faction seeking ashake-up. This may be what’s happening atYahoo: Marissa Mayer doesn’t look muchlike a fierce Bedouin chieftain, but she’sarguably filling the same functional role.

Anyway, the funny thing is that Apple’sposition in mobile devices now bears a strongresemblance to Microsoft’s former positionin operating systems. True, Apple produceshigh-quality products. But they are, by mostaccounts, little if any better than those ofrivals, while selling at premium prices.

So why do people buy them? Networkexternalities: lots of other people useiWhatevers, there are more apps for iOS thanfor other systems, so Apple becomes the safeand easy choice. Meet the new boss, same asthe old boss.

Is there a policy moral here? Let memake at least a negative case: Even thoughMicrosoft did not, in fact, end up taking overthe world, those antitrust concerns weren’tmisplaced. Microsoft was a monopolist, itdid extract a lot of monopoly rents, and itdid inhibit innovation. Creative destructionmeans that monopolies aren’t forever,but it doesn’t mean that they’re harmlesswhile they last. This was true for Microsoftyesterday; it may be true for Apple, orGoogle, or someone not yet on our radar,tomorrow.