Phase 1: Enrolled developers who understand the SDK are scarce, and almost any decent idea sells well in the App store. SDK developers who can execute are king.
Phase 2: There are more SDK developers looking for work, and most of the non-innovative ideas are taken. A good idea that will stand out in the App store, or a hidden vertical market, become king.
Phase 3: The App store gets so crowded that it's hard to make a new or even a good idea stand out. The majority of developers working by themselves sell only tiny quantities of their new apps. Marketing, promotion and branding become king.
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