Given that Unix was primarily a multiuser system in those days, that would be a rarely useful optimisation. Especially if you expected most people to run ed at all times, since it was, after all, the standard editor.
Quite the opposite actually, this optimisation is only useful if you intend to run several instances of the same program. Otherwise each instance has its own copy of the code and rodata, therefore wasting memory.
It's one of the advantages of using shared libraries as well, several programs can share the same code and read-only data, saving in storage and memory space.
