Actually, only private end-user desktop PCs are commonly configured like that where administrator and end-user are the very same person.
In those contexts the distinction between bin and sbin is indeed superfluous.
However, in many other installations the roles of end-users and administrator are fundamentally different and will also be impersonated by different people.
In those cases, the distinction between bin and sbin limits "namespace pollution" of tab-completion for normal users, i. e. tab completion will not suggest executables which cannot be executed by non-privileged users anyway.
Therefore, whether or not /bin and /usr/bin gets merged, bin and sbin should be left separate.