I agree with you that people here are not being very helpful. They're just offering up opinions without talking about specifics of how such a layover could work.

I'll offer a different opinion: While it's true that PVG has not much to do, you can absolutely stay 19 hours. There's food and water. No one is going to kick you out. The lounges may have limited operating hours though so you may not be able to stay the whole time in a lounge. And you also need a boarding pass for your second flight. If you don't have one and need to check in at PVG, then you cannot stay airside.

I absolutely can stay 19 hours at PVG and survive.