Exactly. Consider one observation about Earth life. We know that life started pretty early in Earth's history. It happened within a few hundred million years of Earth no longer being a ball of molten slag. But I can explain that observation with completely opposite conclusions:
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Life may be easy to start. It happened so soon, that life must form nearly anywhere once conditions are sufficient for it. Simple bacterial life is an inevitable process. It's likely our own solar system is brimming with life, with every deep Martian aquifer and outer system ice shell moon overflowing with simple life forms at a minimum.
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Life may be incredibly difficult to start. You'll have to scour a billion galaxies before you find a planet with life as complex as Earth has. But we can only exist on one of those lucky oddballs. The only chance complex intelligent life has of forming on a planet is if everything goes absolutely perfect. If the evolution of life were delayed by a billion years, we wouldn't exist. Within a billion years the warming Sun will boil away Earth's oceans. Our evolutionary time frame is freakishly fast compared to the average. The fact that life started so early is simply a selection effect. We can only exist on a world that evolved complex intelligent life, and that required breakneck pace evolution.
Even this one observation, that life started early, can be explained with completely opposite conclusions. The simple truth is, as you note, we can't know anything about life's prevalence with a sample of only one. That's why we really should work hard to search the solar system for present or past life. It's one of the few shots we have short of interstellar travel of actually determining the prevalence of life.
I'm also skeptical that we'll ever be able to prove life via chemical detections like this one. The problem is that while we may not know of a way for a compound to be produced without life, we can't ever be certain that there isn't some unknown non-biological route for that compound's synthesis. It's an unknown unknown. Maybe dimethyl sulfide can form without life, but in some odd conditions that just don't exist on our planet. We can't prove there isn't some non-biological way to form it. There are really only two ways to prove that a planet has life on it:
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Physically go there or bring back samples. Find said life, examine it with your own eyes or under a microscope, and directly observe it reproducing, reacting to its environment, etc.
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Detect radio or other signals or signatures of a clearly technological origin.
This is why I'm a big proponent of SETI. Even beyond the prospect of making contact, detecting technosignatures is one of the few ways we could have truly unambiguous evidence of alien life. If you find some loud laser or radio beacon belching out long strings of prime numbers, well, you've just proven beyond any doubt that life exists outside of Earth. Maybe that life is long dead. Maybe the life forms were replaced by machines. Who knows. But if you find something clearly technological out there, you can know for certain that there had to be life involved somewhere along the chain from dead random matter to interstellar beacon.
Zambonist.