If we assume for example( note this is fantasy)
If luffy faces and beats kizaru, and then goes and fights kuzan, and defeats him.
While both opponents are of the same strenght, the fight dynamic is completly different. in my opinion there is no need to linear growth of stronger and stronger and stronger, just different fights with same strength.
Oda sometimes, or even usually by this point, builds the fight to have Luffy have some sort of advantage when fighting a (stronger) villain. Back in thevpre time skip we had the lighting immunity with Enel and also the immunity to blunt damage who was used and abused a lot (see Luffy completely fine after taking the punch of Asgard Moria). Now it is a bit already the same thing again: lighting attacks fron Kaido and BM(Zeus) don't work on Luffy, when Kaido is using Boro breath or BM prometheus there us Zoro slicin fire technique and when that is not enough plot power take the stage (like with Zoro stopping a combo attack from 2 yobkos or base Luffy taking a powerful blow from hybrid Kaido when weeks before he was oneshotted in G4 by an almost casual attack from base Kaido..). Or when he stole a top tier skill like FS from an opponent stronger than him (Kata) in the middle of the fight.. Or when Magellan poisoned him, that should have been lethal, of course nobody expected Luffy to die, but still he come out of that with a convenient immunity to poison (which to be honest later didn't work that well, and that is a redeemer).
So for me it is a must that Luffy&co get a bit stronger after each arc. I think both the "base stats" like strength, speed etc. But also coa and coa get a bit stronger plus, for Luffy, in the past arc he gained each time an almighty skill. In WCI he get FS while this time he get advanced coa, most likely in the next arc he will get adv coc plus an increase in base stats again.