“Direct sales cost us just a few percent, so each direct sale of Piezo earns almost $5 more than a sale through the Mac App Store.”

That “few percent” takes into account the cost of our site and our downloads, as well as handling payment.

As far as acknowledging what Apple provides, it's simply not worth very much. The cost of infrastructure is incredibly low (nowhere near the 30% they take), and none of what you list is difficult to manage. Further, any developer who's looking to earn money is going to want to replicate most of that anyway. You should have a website you control to provide information and promote the product (see the many comments about Googling for apps, rather than using the App Store). You also ought to provide customer support for the product itself - Apple certainly doesn't help you there. Once you've got a web site, adding direct downloads is easy. Payment processing and license keys are certainly the most difficult pieces, but anyone making a Mac app can manage them as well. It can all be set up in a manner of days, if not hours. I wouldn't tell a developer that they must steer clear of the Mac App Store, but I absolutely would say they're doing themselves (and their customers) a disservice if they don't also sell their product directly.

re Payments: Have you ever seen how easy Stripe is?

I haven't used Stripe (but have heard good things), but it's still not free. With the App store you don't necessarily need a webpage and all of the costs associated with designing, deploying, and maintaining.