I personally think "user retention" is a harmless sounding euphemism for induced addiction.
To take Netflix for example, it's probably better for them in terms of subscription profits if your "addiction" causes you to watch one episode each month, and not use it at all the rest of the time.
Unless you have non-revenue reasons to addict people, but that's probably headed into conspiracy theory territory... :)
That said, I'm pretty sure netflix has ad revenue, so the point is kinda moot.