I acknowledge your points Bojan, but it is not speculation. Speculation would be talking about some theoretically-modelled occurrence for which there was no empirical data. In this case, we have empirical data (Earth) as well as a rough (though evolving) model of how life formed. It is extrapolation of course.
We started off billions of years ago with a rich cloud of dust and gas, applied the universal laws of physics, and ended up with abundant life. Indisputable, nothing else, no magic. Because life beyond the Solar System may prove difficult (or even impossible) to detect at least in the short term, in considering the possibility of life elsewhere in the Universe we look to other systems. That is, looking for things that might make our Solar System unique. Not so many years ago the existence of extrasolar planets
was speculation. Then improving technology found big ones. More improvements in resolution - smaller and smaller ones. We're still working on resolution, but are down to super-Earths now. Planetary systems are abundant. And that's just close to us in the Milky Way. Talk to planetary scientists and tell them that Earth-sized bodies are pure speculation until proven by hard empirical data!
If you want hard proof before considering the possibility of life elsewhere in the Universe, you may have a long wait. Mars would be great, but really we know already that conditions were right in our Solar System for the emergence of life. I disagree with your view of the way science works - if it was just the interpretation of empirical data, it would not advance.
Cheers -