How do you know it wasn't a demonstration? Do you deny the possibility that intelligent life planted life on earth? If so, how do you know it's not possible?
Well, of course we can't know for certain what happened; perhaps aliens from another planet, one or more of the many gods humans have believed in, or intelligent slime from another universe, or maybe magic pixies. We don't know, and we can imagine an infinite variety of possibilities. Judicious application of known physics can reduce most of those possibilities to 'vanishingly small', but even so, most of what remains is untestable.
All we have evidence for is that there was a time when life - as we know it - wasn't possible on Earth, and soon (geologically speaking) after it became hospitable for life, very simple forms of life appeared; and since then it's become increasingly complex. In other words, the available evidence is consistent with what we'd expect if the simplest forms of life emerged spontaneously when conditions became suitable.
Scientists are curious to know how that might have happened, if what appears to be the case actually was the case; and if a plausible natural means is discovered, that would potentially explain both how life on Earth appeared
and why we've found no evidence of another explanation. It wouldn't mean that we know what happened - there might be many other natural routes to life, or some external intelligence might be deceiving us, etc.; but non-natural explanations would become
unnecessary, or
superfluous; Occam could sleep clean-shaven.
So, until or unless we find evidence that gives us good reason to believe that some external intelligence was involved, it seems reasonable to proceed with the
assumption default hypothesis that there was no
intelligent life on Earth before there was
any life on Earth.
So how long should scientists try to figure out the correct conditions before concluding it couldn't have happened via naturally occurring conditions that were not influenced by any conscious being?
That would require discovering conclusive evidence that a conscious being was involved. As I already said, not (yet) being able to demonstrate something in the lab doesn't demonstrate that it can't or couldn't happen. A look at human history shows that explanations for natural phenomena can take a very long time to discover, and often in the face of an existing consensus that must be overturned (evolution by natural selection, and Einstein's relativity spring to mind).
There have been some lines of abiogenesis research that were shown to be literally dead ends. Other lines encountered apparent road-blocks to progress, such as a proposed reaction sequence that couldn't complete because one step was energetically unfavourable; in at least one such case, it was subsequently found that other chemistry in the same environment produced a simple catalyst that facilitated the otherwise unlikely reaction step. Organic chemistry is incredibly complex, and the variety of potentially suitable natural environments is vast, so we have to be extremely wary of throwing up our hands and saying, "we give up, it can't happen under natural conditions!" - in fact, logically, we can never say this, since we can't exhaust all the possibilities - at best we could say, "We still don't know the answer, but we've decided to stop looking."
But so much progress has been made in abiogenesis research in such a short time, that finding a plausible pathway to simple replicators is looking increasingly likely.
For you personally, what would it take to convince you conscious intent was behind the emergence of life on earth?
That would require discovering conclusive evidence that a conscious being was involved; for example, the indisputable discovery of artefacts in rocks of the period between conditions becoming habitable and the first recognised life, or a vast blue elephant-headed creature moving the stars of the milky way around to spell, "I'm Ganesh, and I created life on Earth!" in every known language, etc.
