well, here's the thing... I don't think this world is so tiny?
I don't think Moana's village is THE ONLY ONE IN THE WHOLE WORLD. Even with the slowly rolling apocalypse, there would be others also trying to tough it out on other islands. Remember that moment in the ancestors scene when we're shown a group of people sailing off while another one stays on the island?
The ocean doesn't depend on THIS PARTICULAR VILLAGE wayfinding again to get people back on it, and I just - I really think it's simpler than that.
Remembering the ways of wayfinders was Moana's personal quest entirely independent from being chosen by the ocean to deliver the heart of Te Fiti - ultimately, I just like this version of the narrative better? Moana is the one who decided where her village would go, and neither the ocean, nor Te Fiti, nor Maui had anything to do with it. She's the daughter of the village chief. It was all her. It's just... aesthetically and thematically neater this way?
Moana has two halves to her identity, in a way - the girl who loves her island and the girl who loves the sea. She was chosen by the ocean, but she also embraced her legacy to lead her people, and I feel like the idea that the ocean also 'chose' her to bring back wayfinding to them cheapens her own agency? Not everything about Moana has to do with the ocean's attempt to fix the world.
(Also, the monsters absolutely existed before the black stuff started spreading - Maui had met the shiny crab before, when he'd had this hook, and that had to be before he stole Te Fiti's heart)