This only happens to big boos main villains though. For example neither Kizaru or saturn can showed anything above Kaido. Saturn can outlast Kaido due to regen but aside from that he was mid. Kaido has better feats than Kizaru.
It depends if loki is the main arc villain of elbaf like doffy or Kaido or not.
But that was not my point.
My point is, every new arc Luffy opponent level became stronger.
I don´t want to start now a debate who is stronger between Kaido and the Gorosei or Kizaru vs Kaido, that was not my point.
Luffy fought Saturn and Kizaru and the other Gorosei.
The challenge was greater as what he fought in Wano.
From fighting Kaido to facing Navy admiral Kizaru and all five Gorosei..
Now Luffy is even stronger than in Wano and Egghead, it wont make sense to jump tiers back and give him someone weaker.
That would be the first time seing in a shonen manga something like that.