Nearly a year ago, I predicted in my Computerworld column that Apple’s iPad would not only eat into netbook sales, but sales of laptops and even desktop PCs. It was an unpopular prediction.

If you look at the 300+ comments attached to that piece, you’ll see that the majority of commenters at the time thought I was crazy, stupid or both.

One wrote: “Obviously Mike Elgan has gone off the deep end on this one. This article is so naive to the real world, and so far fetched it makes me think this is nothing but, once again, a biased article by an iSheep in its purest form.”

Another said: “This article made me laugh out loud. I thought I was reading The Onion!”

Still others were more direct: “I’m pretty sure this is the stupidest article on the internet.”

You still hear people dissing the iPad these days, of course, but nobody dismisses it. Sales of the iPad have far exceeded the expectations of all but a tiny minority of us who were very bullish from the start. Analysts have had to raise and raise again their unit-sales estimates. Early doubters have been silenced.

Now, you might think I’ve come to brag that I was right and my critics were wrong about iPad replacing PCs. A Gartner report published this week says that PC shipments are down from last year. Overall PC shipments in the United States fell by 6.1 percent. HP was down 3.5 percent. Dell dropped 12 percent. And Acer took a nearly 25 percent hit in unit sales. Meanwhile, Apple’s sales grew nearly 20 percent.

One analyst at Gartner said the PC declines resulted from buyers “turning their attention” to media tablets and other devices. The “media tablet” market is a euphemism for the iPad, which owns 70 percent market share and is expected to sell in the 45 million unit range this year.

But no, I’m not here to brag. The replacement of PCs I predicted hasn’t quite begun in earnest. The replacement will come. And I will brag. But for now, it’s more interesting to see how the iPad is gradually undermining the foundations of PC dominance.

Here’s how Apple’s iPad is setting the stage for the decline of the PC.

The iPad is:

Changing the game to Apple’s rules. The iOS platform, and its stunning popularity, is forcing the rest of the industry to play by Apple’s rules. Apple is the benchmark against which other companies attempt to reach. If you want to succeed with any kind of PC, laptop, netbook or tablet, you must have an app store, for example. You have to support lower-powered processors, like the iPad does spectacularly. (Microsoft has always pushed the envelope with new versions of Windows to require increasing microprocessor power. But Windows 8 will support ARM.) You must offer an iPad-like MPG (multi-touch, physics and gestures) user interface. You’ve got to have a retail store. These are Apple’s rules, and the success of the iOS has got everybody playing by them. Everyone can see who’s leading the market right now, and it subtlety weakens Microsoft’s leadership.

Creating uncertainty around the future of Windows. The future used to be so clear. Bill Gates has been talking for fifteen years about how in the future we’ll all use pen-based Windows tablets, voice command desktops and Windows-based mobile devices. By now, everybody was supposed to be using the Ultra-Mobile PC (UMPC) platform for mobile, and Windows Mobile or Windows Phone on cell phones. But that future, so clear in the minds of many PC users, is gone. The only conceivable future for the PC now is one that resembles the future Apple envisioned — MPG desktops, MPG tablets and MPG phones.

Poisoning the well for Microsoft’s “feature rich” credo. Einstein’s definition of insanity is doing the same thing over and over again and expecting different results. By that metric, Microsoft is pretty crazy. Over and over again, Microsoft always shoehorns a too-big operating system on underpowered machines. They did it with various versions of Windows on the desktop. They did it with the UMPC (the vision was Microsoft Vista on an Apple Newton-size device). They did it with Windows CE and Windows Mobile. And now, in the most incredible blunder to date, Microsoft appears to have no plans to right size their tablet OS. Nothing could be more obvious than the wisdom of selling Windows Phone-based touch tablets. The Windows Phone 7 user interface would be incredible on a tablet. Designed for phones, it would be zippy and roomy on a slightly higher power, longer battery life touch tablet, just as the iPad was. But no. Microsoft apparently intends to forgo success again, and cram the ginormous Windows 8 OS onto touch tablets. That’s just nuts. Worse, familiarity with iPad’s performance and battery life has raised the bar for consumers, a bar that Windows is too fat to reach.

Acclimating Windows users to the Apple Way. A poorly appreciated phenomenon is that the vast majority of iPad fans are also Windows PC users. Millions of people are going back and forth between the two platforms, and this growing familiarity is chipping away at the number-one barrier to switching, which is the unfamiliarity and learning curve of a new platform. Many iPad users say they find themselves not only reaching up automatically to their laptop screens to perform iPad gestures, but also accidentally using Apple keystrokes for Cut, Copy and Paste. With each passing day, these millions of Windows users are becoming acclimated to the Apple way. Next time their Dell or HP or Sony laptop is due for replacement, they’ll feel very comfortable considering an iMac or MacBook. One related reason is that the iPad is…

Attracting Windows users into Apple stores. Millions of iPad-owning PC users have reason to visit Apple stores. And each time they do, they’re being exposed to those awesome iMacs, and MacBooks. Apple stores are chipping away at the exotic and alien nature of the Apple universe, in the minds of PC users.

Creating “second-hand cannibalism.” The iPad makes people want a touch tablet. But many people just don’t want an iPad. They hate iTunes, or they loathe Apple’s “closed” platform. The iPad is underpowered, doesn’t support Flash, lacks USB ports, etc. In countless conversations with readers, I’ve found that many just hate jumping on bandwagons. The iPad is a pop-culture phenomenon, and people instinctively hate the idea of being part of that. So they buy an Android tablet. Android is great, and some of the tablets based on that platform are very good. But let’s have no illusions — the huge demand for Android tablets was made possible by the iPad. Google is succeeding in a market that didn’t exist until Apple created it, and their success is based largely on redirected iPad lust. And Android tablet success hurts the PC market, too.

Becoming part of children’s culture. The single biggest way that the iPad is setting the stage for the decline of the PC is that iPad is winning the hearts and minds of children. In a sentiment equal parts true and idiotic, Whitney Houston sang: “I believe the children are our future.” If you’ve got kids, you know that children are obsessed with the iPad. The brand mindshare for iPad on America’s playgrounds is shockingly high, right up there with Disney. These kids will never, ever have any interest in using a mouse. The only way for PCs to succeed with this generation in the future is to copy Apple to the best of their ability.

Gartner’s report does not prove that iPads are “cannibalizing” the PC market. But make no mistake, the beast is going in the oven and the feast is coming.