Are you really suggesting that people buy newspapers based on the newspaper's political leanings?
Look at it another way. Suppose the Mail, Express and Sun all developed more left wing tendencies and printed more about socialism. Not immediately, but say gradually over a long period. Would most of the regular readers of those papers stop buying them because they were no longer right wing, or would most of them continue reading the same papers but change their own political opinion in tune with the paper they read?