I believe people are over conflating the "friction" involved with using an outside payment system (such as Stripe or PayPal). Apple ID's already allow you to attach your PayPal account as primary funding...this is just giving developers that direct choice now.
PayPal, you confirm checkout total, login to paypal, confirm subscription or price. Done.
Stripe, you can use their standard checkout page, autofill your card info, or just use Apple Pay to confirm the subscription/item, pay. Done.
What's changed is just giving developers that flexibility. Ultimately saving them money, they can hire more devs, and make their products hopefully cheaper (and better).
Most consumers will still have no idea that their checkout is not happening with Apple, and it's happening elsewhere (aside from PayPal Checkout being obvious with their checkout/login flow).
Apple could absolutely adapt their native subscriptions SDK to support the status of a third-party app, though I doubt they ever would. They tried to do this with streaming services (HBO, Netflix, etc.), but they shut this down recently