Without constant replacement it's gone pretty fast.

Most things have maintenance costs. If Starlink makes enough money to fund the required satellite replacement rate, that's sustainable.

Sure, but that's true of a lot of business models, many of which have proven to be quite sustainable. Grocery stores, for instance.

Which makes it sustainable... if it goes away shortly after you stop replacing it, it's not putting debris up there.

Needing constant rocket launches to provide a service isn't sustainable, Star Link is disposable

If we're talking about economic sustainability, that's not how it works. Many things in your daily life are disposable (remember this point for the next paragraph) and yet are economically sustainable for the companies making them because they bring in more money than they cost.

If you're talking about environmental sustainability.. get over it. Rocketry, even as SpaceX's scale, is a tiny drop in the ocean of throwaway consumer culture.