It's obviously not something that is going to happen, but it's also not that crazy of an idea. They use more water so you pay for more water. They use more clothing so you pay for more clothing. They use more cellular devices so you pay for more cellular connections. They each want an email address so you pay for more email accounts. They use more digital content so you pay for more digital content.
I wouldn't put it like that. I would say that each user using Netflix should pay the same amount. So you pay for yourself, your wife pays for herself, and your kid pays for themselves.
Sure, it's 3 times more than what you pay today. But you are 3 times as many people as I am, so why shouldn't you?