They are best thought of as circumstantial evidence, especially when combined with the various other possible indicators we have detected on Mars. Many books I have read over the years talk about the Viking experiments as if they had disproved life, whereas they did no such thing.

However, I think the bulk of life on Mars – if it exists – is probably underground. Conditions there are much less hostile than the immediate surface. Since Viking, we continue to find Earth life in extreme environments including high altitudes and deep underground. Microbes can be much tougher than animals.