WWE was gonna lose many of its fans anyways(the ones who only started watching in attitude era,cause wrestling was "cool).
So WWE sorta sped up the process, and thought about their future. Switching to PG, WWE knows that they'd lose some of their older fans(but like I said,it'd be most of the fans who'd leave eventually anyway,once guys like Taker,HBK,Jericho all retired), but they'd also replace those fans with younger fans who'd continue to watch WWE.
Ask most people, when did they first start watching wrestling, many would start talking about watching it when they were a kid. That's what WWE is doing now too. If you watch something(such as sports) as a kid, chances are, you'll be less likely to grow out of it, then if you were to start watching when you get older.
The population also has more people 0-18 than 18-40, so it's a bigger target audience by targeting kids.
The only bad thing I see about WWE being PG is the content itself. Not cause PG restricts stuff, but cause WWEs PG restricts stuff. There's so many things WWE can do that IS allowed on PG, but they don't really do it(on recaps,they show blood in black in white. I understand WWE not wanting to have blood anymore for safety reasons, but blood IS allowed to be shown on PG)
You can be PG-DLSV, WWE is only PG-V. Meaning there is DLS that can also be added to their PG rating(if you don't know what PG-DLSV is,look it up)