Luffy become Yonko after Kaido, according to your logic Luffy should've become PK as Zolo becomes WGS. So this alone should debunk your argument.
Also Luffy didn't power up vs Doflamingo.
Zolo powered up vs Alber.
Again this debunks it.
Aokiji (who is Akainu's equal) is in BB pirates and Shiryu as confirmed no.2 is around Admiral level.
There should be no power difference between Shiryu and Admirals. Again this debunks the idea of Shiryu being training for stronger opponents.
Also, Shiryu and Mihawk are in the same positions, both are Yonko underling.
Katakuri and Kaido were not in same position. As for WGS should put Mihawk above, even the skill difference between them isn't close, Shiryu who was equal to Magellan by only swordsmanship (I doubt they would say Vista is equal to Magellan if Vista was head jailer in Impel Down), Shiryu has still a powerful DF to balance it and once awakens it, it will be on different level.
Another thing is, Mihawk trained Zolo, Kaido didn't train Luffy.
Zolo and Mihawk can never be rivals like Luffy and Kaido after Mihawk trained Zolo for 2 years, now they are more like friends rather than rivals.
Its better if Mihawk loses to someone before they fight, so Zolo surpasses them.
Also Luffy didn't power up vs Doflamingo.
Zolo powered up vs Alber.
Again this debunks it.
Aokiji (who is Akainu's equal) is in BB pirates and Shiryu as confirmed no.2 is around Admiral level.
There should be no power difference between Shiryu and Admirals. Again this debunks the idea of Shiryu being training for stronger opponents.
Also, Shiryu and Mihawk are in the same positions, both are Yonko underling.
Katakuri and Kaido were not in same position. As for WGS should put Mihawk above, even the skill difference between them isn't close, Shiryu who was equal to Magellan by only swordsmanship (I doubt they would say Vista is equal to Magellan if Vista was head jailer in Impel Down), Shiryu has still a powerful DF to balance it and once awakens it, it will be on different level.
Another thing is, Mihawk trained Zolo, Kaido didn't train Luffy.
Zolo and Mihawk can never be rivals like Luffy and Kaido after Mihawk trained Zolo for 2 years, now they are more like friends rather than rivals.
Its better if Mihawk loses to someone before they fight, so Zolo surpasses them.