Because Luffy was there to keep him in check, without Luffy being there Oda will find another way for them to manage. If you dont think so then idk what to tell you lol.
Stop making generalized arguments, we already have our answer to this. Oda is actively keeping Kizaru in check rn, by having him stay down. So he did find a way for them to manage, we are seeing it happen as we speak.
Before Kizaru killed VP, Luffy was actually
not there to keep Kizaru in check. Oda forced Luffy to come back just to do it. That was because Oda wrote Kizaru to be too much for anyone else to handle. That was
Oda who did that, and by default he cornered himself. With no Luffy in the equation he would have to come up with another answer, and thus we are in the situation we are in now. Kizaru is
still down, because of emotions because Oda is not interested in Luffy being locked into combat atm.
Again, plot armour has existed in One Piece since the very first chapter. Stop crying about it now.