I don't think it was weak at all. We got two positive results from the Viking landers and then tried to explain them away. There is a lot of circumstantial evidence, which combined builds up a case.

The public believe we should be looking for cities and skeletons, when in fact we should be looking for extremophiles. It is possible that some of these are due to cross-contamination within the solar system (before human intervention), but that is another matter.