Do you mean the early date of P 52 or the early date of the autographs of John?
I have not found any protests regarding the date of P 52. There is consensus among C. H. Roberts, Frederic Kenyon, H. I. Bell, Adolf Deissmann, and W. P. Hatch. I have found the arguments very convincing for the early date of the papyrus. P52 shows critical similarities to many other early papyri: P. Fayum 110 (94 CE); the Egerton Gospel (130-50 CE); P. Oslo 22 (127 CE); P. London 278 (81-96 CE); and P. Berolinenses 6845 (ca 100 CE). The closest papyrus is P. Berolinenses 6845 (ca 100 CE) - even P. Berol. 6845 has handwriting that parallels other first century papyri, but the editors dated it second century because of its appearance. Therefore, the editors of the most recent publication of P52 have dated it closer to 100 CE (Comfort, The Text of the Earliest Greek NT MSS, 2001).
The dating of P52 has been used to date the Gospel of John. Bultmann, for example, wrote an influencial commentary on John. He believed that John was known in Egypt by 100 CE because of P52. This could possibly push the dating of the Gospel back to 85CE.
However, P52 by no means dictates an early date for the authorship of John. It simply means that some of the traditions in John were widely known at an early date. We can argue, with the majority of scholars, that the writing, editing, and compiling of John continued well into the fourth century.