No, actually, you don't. And Android is not closed source. Google's add ons are.

When you buy a used phone, it's already been bought by someone. The profit is already in Google's hands. Loads of old phones are in drawers somewhere. Once Graphene is on it, it's faster, not bloated, and you don't need Google anything.

You could offset that by convincing others to use graphene, and by degoogling your device you’re also cutting off one of their other income streams