A merge needs to be done carefully for backwards compatibility.
You could move all the things in /bin and /sbin to /usr/bin and /usr/sbin, then leave behind links (symbolic or hard).
Since everyone ends up having /bin and /usr/bin in PATH, this merge makes a lot of sense from a performance point of view.
Merging bindirs and sbindirs is a touchier topic. Many things in sbindirs should have been in bindirs all along, and many should move to libexecdirs, but some should stay behind so that privileged users can keep sbindirs out of PATH when they're not wearing an admin hat.
You could move all the things in /bin and /sbin to /usr/bin and /usr/sbin, then leave behind links (symbolic or hard).
Since everyone ends up having /bin and /usr/bin in PATH, this merge makes a lot of sense from a performance point of view.
Merging bindirs and sbindirs is a touchier topic. Many things in sbindirs should have been in bindirs all along, and many should move to libexecdirs, but some should stay behind so that privileged users can keep sbindirs out of PATH when they're not wearing an admin hat.