This is a specific instance of a general phenomenon. Logical ethical arguments only make any sense in the context of a specific ethical system, just like mathematical logical arguments only make sense in the context of a specific set of mathematical axioms. Every ethical system prefers itself over all alternatives (by definition in its opinion the others all get at least some things wrong). So any time anyone makes what sounds like a logical ethical argument for preferring one ethical system over another, there are only three possibilities: their argument is a tautology, there’s a flaw in their logic, or it’s not in fact a logical ethical argument, it just sounds that way (normally it’s in fact an emotional ethical argument argument). (The only exception to this is pointing out if an ethical system is not even internally logically consistent, i.e. doesn’t make sense even on its own terms: that’s a valid logical ethical argument.)
If that didn’t make sense to you, try the first four paragraphs of the first post in my sequence on Ethics, or for a lot more detail see Roko’s The Terrible, Horrible, No Good, Very Bad Truth About Morality and What To Do About It. You cannot use logical ethical arguments to choose between ethical systems: you’re just pulling on your own bootstraps if you try. If you don’t want to just pick an ethical system arbitrarily, you have to invoke something that boils down to something along the lines of “I do/don’t feel good about this rule, or its results, or I’m going to pick an ethical system that seems fit-for-purpose for a particular society”. So basically the only way to make a decision about something like SCEV is based on feelings: does it offend the moral instincts that most humans have, and how would most humans feel about the consequences if a society used this ethical system (which generally depend a lot on what society we’re talking about)? So you do need to think through consequences like providing vegetarian meals for predators and healthcare and birth control for insects, before picking an ethical system.
This is a specific instance of a general phenomenon. Logical ethical arguments only make any sense in the context of a specific ethical system, just like mathematical logical arguments only make sense in the context of a specific set of mathematical axioms. Every ethical system prefers itself over all alternatives (by definition in its opinion the others all get at least some things wrong). So any time anyone makes what sounds like a logical ethical argument for preferring one ethical system over another, there are only three possibilities: their argument is a tautology, there’s a flaw in their logic, or it’s not in fact a logical ethical argument, it just sounds that way (normally it’s in fact an emotional ethical argument argument). (The only exception to this is pointing out if an ethical system is not even internally logically consistent, i.e. doesn’t make sense even on its own terms: that’s a valid logical ethical argument.)
If that didn’t make sense to you, try the first four paragraphs of the first post in my sequence on Ethics, or for a lot more detail see Roko’s The Terrible, Horrible, No Good, Very Bad Truth About Morality and What To Do About It. You cannot use logical ethical arguments to choose between ethical systems: you’re just pulling on your own bootstraps if you try. If you don’t want to just pick an ethical system arbitrarily, you have to invoke something that boils down to something along the lines of “I do/don’t feel good about this rule, or its results, or I’m going to pick an ethical system that seems fit-for-purpose for a particular society”. So basically the only way to make a decision about something like SCEV is based on feelings: does it offend the moral instincts that most humans have, and how would most humans feel about the consequences if a society used this ethical system (which generally depend a lot on what society we’re talking about)? So you do need to think through consequences like providing vegetarian meals for predators and healthcare and birth control for insects, before picking an ethical system.