A satisfactory solution should also work if the em population explodes to several quadrillion. As I said, 'all sorts of questionable issues'; it's a rule-of-thumb to keep certain troubling edge cases from being quite so troublesome, not a fixed axiom to use as the foundation for an ethical system that can be used by all sapient beings in all circumstances. Once I learn of something more coherent to use instead of the rule-of-thumb, I'll be happy to use that something-else instead.