I don't care vs who cares

I don't care

phrase
  • Indicates that the speaker has no interest or emotional investment in the topic at hand. 

who cares

phrase
  • A reply to an unimportant or irrelevant statement, indicating indifference on the part of the speaker. 

  • A reply that diminishes the importance of another speaker's immediately preceding statement. 

How often have the words I don't care and who cares occurred in a corpus of books? (source: Google Ngram Viewer )