Postbox’s exit from the Mac App Store should sound very familiar to anyone who buys Mac software. If you read between the lines a bit, I think the real story there is one we’ve seen a lot since June 1: they tolerated the App Store’s lack of paid upgrade mechanics before, but sandboxing — and more accurately, needing to remove important app features because of their incompatibility with the current set of sandboxing entitlements — was the last straw.

How many good apps will be pulled from the App Store before Apple cares?

The problem with sandboxing isn’t that any particular app is incompatible with the current entitlements. It’s a deeper problem than that: Apple is significantly reducing the number of apps that can be sold in the Store after people have already bought them.

Apple’s stance seems to be pretty typical of them: comply with the new rules or leave. This usually works for them, but this time, they’ve made a critical strategic error: leaving is often a better option, or the only option, for the affected developers. Many of them have already left, and many more will.

In the first year of the Mac App Store, before sandboxing, I bought as much as I could from it. As a customer, the convenience was so great that I even repurchased a few apps that I already owned just to have the App Store updates and reinstallation convenience. And, most importantly, when an app was available both in and out of the Mac App Store, I always bought the App Store version, even if it was more expensive.

But now, I’ve lost all confidence that the apps I buy in the App Store today will still be there next month or next year. The advantages of buying from the App Store are mostly gone now. My confidence in the App Store, as a customer, has evaporated.

Next time I buy an app that’s available both in and out of the Store, I’ll probably choose to buy it directly from the vendor.

And nearly everyone who’s been burned by sandboxing exclusions — not just the affected apps’ developers, but all of their customers — will make the same choice with their future purchases. To most of these customers, the App Store is no longer a reliable place to buy software.

This jeopardizes Apple’s presumed strategic goal of moving as much software-buying as possible to the App Store. By excluding so many important apps and burning the trust of so many customers, the App Store can never become ubiquitous.

Apple can never require an App-Store-only future and all of the simplicity and security benefits that it could bring, if that was ever their goal. And with reduced buyer confidence, fewer developers can afford to make their software App Store-only.

This even may reduce the long-term success of iCloud and the platform lock-in it could bring for Apple. Only App Store apps can use iCloud, but many Mac developers can’t or won’t use it because of the App Store’s political instability.

The Mac App Store is in significant danger of becoming an irrelevant, low-traffic flea market where buyers rarely venture for serious purchases. And I bet that’s not what Apple had in mind at all.

Follow-up to this post: It’s not just the geeks like us.