Loki was also a side character till this arc, and Yamato was alone fighting Kaido who a lot of people had as top 1 of the verse by the time they fought
Comparing Yamato, a side character, to the Main Character, which she herself admitted was stronger than her and comparing Loki with 2 other side characters is not the same.
Yamato herself said Luffy is the one beating Kaidou - nobody thought she was stronger than fucking Luffy....the only discussion there were is if she was stronger than Zoro and Sanji.
Putting Loki on "PK tier" is pure assumption and headcanon till proven otherwise
Harald = Rocks -> Harald got HK boost -> Loki beat Harald... -> ''Headcanon''
This one shot thing to me isnt really a point when Luffy got one shot by Kaido at the beginning of Wano then proceeded to beat him at the end of the arc
After the one shot Luffy, the MC and the strongest in the crew, trained for 1-2 weeks, died mid fight just to unlock his awakening of a God Fruit.
Zoro already got CoC and used it against Imu, was easily blocked and was one shoted. What kinda PU is he supposed to get to go from getting one shoted to what Loki, who also has a God Fruit and a mythical Zoan weapon, is doing against a being that is far stronger than Top Tiers? The only chance they have is reaching Loki's lvl EOS and even that is almost unlikely. Let alone in the near future.
Whether he joins or not, its another discussion, but i dont really see any problem with that, it would be like Oden joining Roger's crew
Oden was weaker than Rayleigh and Gaban, unlike Loki whos levels above Zoro and Sanji.