It is about them being the first ones.
Like if you started watching WWF you're not going to watch TNA just because its wrestling, if you get sick of WWF then it pretty much means youre sick of wrestling.
But netflix is constantly making moves and the more people that sign up will just increase their residual income stream.
Yes many people use the same account but a large portion of those people will end up paying for the service themselves.
Also there's millions and millions of upcoming new accounts as more and more people buy new TVs and devices that have netflix built in and more younger generation move out and get their own bills.