It's fine if you don't like it, but Nika was teased as far back as Skypiea and then in Thriller Bark Oda speaking about Luffy being Dragon's son, means Oda probably had Kuma seeing Luffy as Nika planned out.
It's fine to hate Gear 5 and Nika because it is a very contoversial decision, but there are a lot of panels from Skypiea that proves Luffy has been set up as a true god who frees people fighting false gods who enslave people.
Everyone is praying for god to save them, and Luffy saves them while shouting "Let the sun shine!"
Nika isn't a retcon, it just took fucking forever to get to it, and it's a very controversial choice.