Nightmare luffy was never luffy’s power to begin with
A Kurama type of thing but only one use.
Especially if it is as powerful as Nightmare mode was.
In that case, it's going away 100%.
Imagine Luffy proceeding with Nightmare mode, lol...
Joyboy isnt necessary for Luffy to fight top tiers.
Even weaker characters than Luffy can fight top tiers.
The more powerful Joyboy is, the higher the chances it goes away.