Per Blizzard’s own poll HERE, Retail is at 56% and Classic is at 41%. Forum polls are flawed in that not all players visit the forums, but it’s the only objective evidence we have since Monthly Active Users (MAUs) in ActiBlizz SEC financial filing are never broken down by game mode (only by Divisions among Activision, Blizzard, and King).
Subsequently those that think Classic is more popular than Retail are making a baseless claim; that is without any objective evidence, only opinion.
While I think @Kiper’s comment about shutting down Classic being wrong, is a valid statement, we don’t share the reasoning for it being wrong: They think it’s because Classic is more popular but there’s not great public evidence to support that. I think it’s because it would be a revenue loss to Blizzard if they shut it down (it’s probably true that many Classic players have no interest in Retail).