How can anyone "see it?"
We don't watch Dodger baseball wire-to-wire.
I'm going off the notion that JBJ has declined, and I think there is ample evidence to show he has, despite a 55 game blip upwards, last year.
I'm also going off a 1200 inning sample size of EHern in CF, which surely can be doubted and criticized.
My position is based on the last 3 years' trend and the fact that JBJ turns 31, soon and EHern is 29.
I'm never going to say EHern is clearly a much better defender in CF than JBJ, at this moment in time, but his defensive metrics show him better, his OPS+ is better over the last 3 years, and his bWAR per 650 PAs is better over the last 3 years. I think the burden of proof lies with the JBJ supporters.
We've all watched JBJ play great D for many years. We've watched flashes of a great bat that have basically kept his overall offensive numbers near average or even above for extended times, but he's not getting better, whereas EHern seems to be.