An existing customer base can be a moat if there's an effect from scale, like a game where you need there to be people on a server to make it fun, sure. In the case of an app like Figma though having more customers than someone else is irrelevant to the product.

If someone can make a passable clone that does the part they need they have no reason to stay on the product, so they won't. Even the most popular SaaS products are facing a very real threat from AI unless they're either something that benefits from more users, something in a regulated industry that people can't just run even if they can copy it, or something that has a secret sauce that can be kept a secret.