Thats expected to happen, the story is close to ending of course Luffy would be getting close to become the strongest character of the verse.
Cant see the problem with he becoming another Kaido/BM, what u guys wanted? He to keep being a Doffy-lvl when he get to Laftel?
And for the next enemies he need to be top tier, the only ones Luffy will fight after Wano are guys like Akainu, Teach, Shanks and Imu.