Unification isn't the only way it would work because sometimes a champion might not even be better than a contender. Such as Joseph Parker being arguably inferior to Kubrat Pulev or Alexander Povetkin. And one of the champs may have only been able to gain their title due to being lucky by perhaps fighting for a vacant belt which one of the other contenders who are even better as a boxer may not have had the luck and the opportunity to fight for. Thus, beating a champion and unifying the title doesn't necessarily mean you are the lineal champion because you are still yet to face another boxer who may not be the champion, but is superior as a boxer to the champion that you beat.
Thus, the only way to become the lineal champion is to clean up the division. And to do this, beating at least one other champion to possess most titles in the division, along with defeating at least 10 mandatory challengers ranked in the top 10 is more or less akin to cleaning up the division and thus being qualified to earn the 'lineal champion' title.
Unless you have a different criteria to determine when a boxer may have "cleaned" up their division or you disagree with cleaning up the division being sufficient enough?
Thus, the only way to become the lineal champion is to clean up the division. And to do this, beating at least one other champion to possess most titles in the division, along with defeating at least 10 mandatory challengers ranked in the top 10 is more or less akin to cleaning up the division and thus being qualified to earn the 'lineal champion' title.
Unless you have a different criteria to determine when a boxer may have "cleaned" up their division or you disagree with cleaning up the division being sufficient enough?
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