Bro, deal with it, stop this mindfuckery.
It's a shonen manga, the strongest is the captain, the second strongest is the vice and his right hand.
Nothing more nothing less.
No seniority, no age, no attitudes, nothing... only power matters.
- The next will OBVIOUSLY be Katakuri!
- No, we operate on seniority...
- NONSENSE! POWER IS ALL THAT MATTERS!
Katakuri, not Perospero.
Power matters to be captain or vice captain, not age, seniority, roles, attitudes and what not.
Plus the cover with all the number twos , aka the second right after their captains, in both power and command. In that cover Oda did put Katakuri, not Perospero.
There's literally nothing more to say.
It goes for Katakuri, it goes for King, for Zoro, for all of these characters.