I see it as the reverse. Those who constantly say the Spurs can't win are just playing the numbers so they can shout "I told you so!" and wax their anti-player X agendas, should the Spurs fall short, and, if the Spurs do win they know everyone will be too elated to call them out on it, and, even if they do it will be lost under the euphoria. It happened with all the doom sayers after 2013, who were all too happy to celebrate 2014.
A certain poster (who shall remain nameless), for example, circa 2011, made the bold prediction that the Spurs would never win another championship with Manu on the team. In 2014 he was celebrating along with everyone else, with not a word about his prediction.
Since no franchise wins the le nearly every year since the 60's Celtics, the naysayers and nilists get to feel "right" most the time, they know that game, while others are talking about how the Spurs COULD take the LOB in any given year, even if the odds are stacked against them. I guess those with a positive at ude are vanilla.