If CocoaPod was run by a business that was charging those millions of developers for their services, it would be reasonable to expect that business to pay for a real CDN.

They're not, so that's not a reasonable expectation. But it's no more reasonable to make these demands of GitHub. Giving out a free product doesn't mean that you're required to give it out unconditionally, or in unlimited amounts, or forever. In the end, GitHub owns the infrastructure and services it's providing and can do what it wants with it.