Congratulations on a well-reasoned article, Matthias. Personally I think you've made a number of logical errors (and you've inspired me, perhaps, to write an article that covers them in detail) but you've done an excellent job of showing the logical fallacy of what one may term the "naive" argument for extra-terrestrial life. One thing you didn't cover is the habit people have of conflating life with intelligent technological life. While I think there is in fact a reasonable case to be made that primitive life (single-celled) is likely to have arisen elsewhere by extrapolating from what we currently know about biological and non-biological systems, there are extremely strong arguments against the notion this would inevitably mean the presence of another technology-capable species located anywhere near us in time and space. Thus the search for ET is pointless even if life is abundant elsewhere.