They'll surely lose some subscribers in the short run, but it's not clear to me that it'll be a net loser in the long run. There are people who share accounts because they are very price-sensitive, and they'd be more likely to bail and find other ways to get content (competitors that aren't cracking down, or pirating). But there are also people who share accounts for historical reasons, like grown children and their parents. Those folks may have more than enough disposable income to have their own accounts, or pay a surcharge for having a joint account.

The question is: what percent of NFLX's multi-household subscribers will (1) quit, (2) pay a surcharge, or (3) get a separate account? My guess is there aren't a lot in (1), most are in (2), and there are very few in (3). I imagine they're trying to learn from their experience with this pilot program so they can figure out an appropriate surcharge.