Presumably they know when they are streaming for a given account. They could simply say an account can be used for a single stream, and decline to start a second concurrent stream for the same account. Include download playbacks as a form of stream. The headache of coordinating watch times when sharing accounts would incentivise additional subscriptions.
That is precisely what they shouldn't do. I have several televisions and devices in my one house.
Granted... We don't actually watch too much television, such that it may not actually impact us if they do it this way. Still, I imagine many families with children would be heavily impacted by this strategy.
