The distinction is that Mastodon doesn't have to grow, and Bluesky does. There's an extent to which Bluesky is viable because its venture-funded owner pays for the infrastructure it operates on (Mastodon communities generally have to stand on their own two feet). If there's no path for Bluesky to grow into a viable business on its current financial runway, the funding for the infrastructure will end.
We already have others running infrastructure as well so if bsky goes away the platform can survive
ATProto infrastructure is mostly open and there are folks operating pieces of it independently even if the landscape is dominated by the Bluesky PBC. It's true that Mastodon grew bottom-up without centralized funding so the shuttering of the PBC will be a big deal for the AT ecosystem but it's all independently doable.