What if we farm the isolationists? Sentience has a fuzzy definition, and by time we meet alien life they may be more similar to animals than humans. Maybe they use some tools from time to time, communicate with each other, have families, or some other things, but otherwise resembles animals. Would it be "right" to, because they are clearly incapable of resisting, put some in a zoo and farm the others for a valuable resource (say, their bodies produce diamond from carbon)?
Crimes of ignorance can be forgiven, I think. But to avoid this, we should find a solid definition of sentience. (note, this may mean we're not the only sentient species on the planet)
On the topic of sentience, I'd put forth Theory of mind as the key qualification of sentience. It can be tested for, and most animals do not have it, only some great apes (including homo sapiens of course) and interestingly, dolphins exhibit it.
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eeyup, they're sentient alright