JBL is shit because JBL is shit. No one tells him to say the same shit over and over again, he does it for his own autistic amusement.
You're honestly telling me that the drop in quality of JBL's commentary when he first came back vs. the one now, even if he regressed, is natural? Everybody knows the commentators get fed shit by Vince. Even in this thread its been mentioned that new commentators will mean fuck all unless Vince gets out their ears.
Cole is amazing on the microphone. Of course his commentary is restricted and controlled, but it's extremely articulate and well composed when he does interviews. Watch some of his interviews in old WWE videos or broadcasts.