Krashens hypothesis is just that people acquire languages by understanding messages. Not by studying grammar, memorizing vocab, and "traditional" learning (IE based on skinner's method, of error=punish, correct=reinforce).
Not only is CI backed up by evidence, and by the many polyglots who have successfully learned many languages through CI / immersion, you'd also need to show evidence of babies not learning their first language this way to refute it (IE show evidence of babies learning their first language by studying grammar and doing flashcard study).