PayPal doesn't have to worry about being replaced as much as it has to worry about being marginalized. If a developer who has never integrated payment into their app asks me what platform to use, I'm going to answer "Stripe" without hesitation because they make it very simple for developers to use. PayPal needs to shift its focus toward making the developer experience better, because right now their documentation is a steaming pile of shit and their web interface is unintuitive.