it's not that they don't make money, but they don't allow developers to operate good businesses. being able to give trials to users, or to handle advertising in certain ways, or to have more flexibility with payments, etc - all of these things would be beneficial for businesses to be able to control directly, but they can't, as they're in the walled garden.

Many are making money right now, but I would suggest they don't have terribly good businesses, in that they don't own the relationship with the end user. One change of Apple's policies can put you out of business.

How about comparing instead to money made by Apple, enabled by iOS developers?

It sounds like you feel Apple owe developers a living?

Apple could attract more developers (and thus more iPad users and more corporate revenue -- reversing declining iPad growth) if developer success was more closely aligned with Apple success.

Attracting more developers does nothing to attract more users.

How about attracting financially successful (not hobbyist) developers who can drive new use cases for the platform?