Oh I have no doubt that all sorts of fashionable theories and speculations come up from time to time...no doubt.

But John wrote his Gospel.  He was the beloved disciple.  He wrote it as if he were an eye witness adding in many details..."behold your mother..." "I thirst".

And the early church, and his peers didn't reject his eye witness account when he wrote it about  90. 

The "skill" argument is preposterous.   Many great writers had no training or credentials.  That's an elitist argument. 

Many great mystics write with such apparent ease and beauty that the Church considers their writing inspired.

The more complicated and multifaceted your theories become, by parsimony, they should be rejected.