Netflix has been enforcing concurrent streams for a while now, no?
Four adults that live in four different houses sharing one Netflix account might not ever bump up against the four concurrent stream limit. Their concurrent usage might be quite similar to a family of four under one roof.
But critically, the four adults are all technically capable of paying for their own subscriptions, while the children of the family of four are not.
Netflix is expecting to lose some (or most) of the people who are sharing passwords, but if some nonzero portion of them end up coughing up money for their own subscription, the effort is worthwhile.