Re: "Speculation in absence of evidence is futile"
We have a planetful of evidence. This is, after all, a planet, in the universe, which, so far (until or unless we render it uninhabitable), supports life.
Every scientific hypothesis goes beyond what we know, by definition. The difference is that exobiological hypotheses will probably never be proven or disproven.
However, that's probably going to prove equally true for much of the speculation going on today in physics/cosmology. Even the large hadron collider at CERN can't remotely approach the Planck scale. So, in effect, our best microscopes can't see small enough, and the energy it would take to get to that scale is beyond anything we can imagine possessing. We can extract useful information about subatomic reality from astronomical observation, but again that can only take us so far. Likewise mathematical inquiries have gotten us down the pike, but warring mathematical models will remain at a standoff absent experimental evidence.
Yet physicists continue to give it their best shot.
Re: "Our diversity proves anything's possible"--words to that effect
Our only example of life--this biosphere--shows stunning diversity, and as a veteran scuba diver, I've seen more of it than most.
But this kind of argument fails to address the innumerable examples of evolutionary convergence found in every biotope. A rejoinder of this sort that doesn't address evolutionary convergence is reductionist.
Not to mention the Assumption of Mediocrity, which, while hardly a law of nature, has proved to be a useful starting point for many speculations and hypothesizations.
And here we have a science fiction/fantasy forum.
Well, in fantasy you can cook up whatever you please. Dragons, magic, yada yada. You just to need to stay true to your universe.
OTOH science fiction isn't fantasy--it's supposed to extrapolate from what we know. That's speculation, and that's why it's also called speculative fiction.
So if someone writes a science fiction story with no basis in science for what they're saying, that's fine as long as they admit that what they've written is fantasy, not science fiction. Likewise if the only basis is other science fiction stories or movies--or movies in general--again, that's fantasy...of a rather inbred sort.
Thus the Star Wars cantina scene is based on putatively science fiction movies George Lukas saw as a child, and has in turn given rise to biologically absurd assumptions on the part of the public at large.
Much if not most of the biology found in science fiction lacks an evolutionary model. It doesn't so much extrapolate from what we know as directly contradict it. OK. Just call it fantasy.
Bottom line: our own world gives us plenty of clues about how life can and can't evolve. I'm simply proposing that we think about what we've discovered about biology instead of just throwing one black box after another at the problem.