A huge portion of iOS' ongoing success relies on the quality / quantity of apps that are exclusive to iOS. That exclusivity is in serious danger if/when devs move to cross-platform HTML5 web apps. And if anyone has the sway to move developers in this direction, it's the dev with the largest user base on the planet - Facebook. Now that they are embracing HTML5 web apps, many others may well follow suit.

Facebook moving to HTML5 will get tens of millions of iOS users comfortable with using an HTML5 web app, rather than a native one found in the Apple walled garden. This changes the game dramatically, and not in Apple's favor at all. Web apps never really took off because they've almost always been glorified web sites with limited functionality and utility. HTML5 is powerful enough to change all of that. If end users embrace HTML5 web apps, the complexity, cost and artificial restrictions of programming native apps will seem positively wasteful and unnecessary for 80% of the apps out there.

There's already huge incentive for devs to go down this road - HTML5 web apps work on every mobile platform out there, as well as desktops, set-top devices etc. You can truly build it once and deploy it everywhere, without a dime of app/in-app purchase revenue sharing, and you retain total control over your content. This is Apple's worst nightmare come true in many ways.

Here's another angle - when (not if) Adobe incorporates HTML5 output into InDesign, magazine publishers will have almost zero incentive to participate in Apple's magazine subscription scheme. They can just release issues as HTML5 web apps that work on every tablet/smartphone out there, leveraging the exact same assets as the print issue, they keep every dime of revenue, and maintain control over their user database which is a huge point of contention for them right now.

The only downside in all of this is that devs will lose the "free" promotion that the app store provides - but it's not free at all since Apple takes a 30% cut of sales for said promotion. It should also be noted that most devs gain zero visibility from being in an app store with 5 million other apps, unless they are one of the chosen few to be featured. HTML5 app devs will make an extra 30% revenue from their apps, and that money can be spent on marketing of their own choosing. My guess is that average devs are actually MORE likely to make money from their apps under this system than the app store ever provided them.

None of this is good for Apple at all. I bet Steve Jobs wishes he would have kept his big mouth shut when he used HTML5 as a bludgeon against Adobe Flash. Chickens coming home to roost and all of that...