Apple

So how would you pay for the ongoing development of developer tools, store infrastructure, backend development and upkeep of the ecosystem, etc.

They literally net a billion dollars a week profit just from selling the iPhone itself. On top of that they net another billion dollars a week profit from all their services and other hardware. If you subtract their profit from the App Store, in the unlikely event it was entirely wiped out by competition, they would still have 2/3 of that or about $1.4 billion a week profit after all expenses. They would be just fine, making massive profits off iPhones after all iPhone-related costs, even if they lost the App Store entirely.