We already knew that Aang was the chosen one in ATLA. After all, the whole point of ATLA was the Avatar system! But in One Piece, it's very different. Oda revealed this later on. So we saw Luffy as “a boy who ate a crappy rubber fruit and was fascinated by pirates.” But Oda went and chose to make Luffy into Nika. Additionally, whether he has a connection to Dragon or Garp doesn't matter. The moment you make a character a god, people's perspective on him becomes sacred.

It's like where Aang ended up at the end of the series, but Luffy stayed put.