Hence why my last paragraph contained ways of putting restrictions in place that'd make it less practical/desirable to keep sharing. (Here's one more suggestion: Add premium content, and let anyone buy from any profile... Now you have a strong incentive not to share with people you can't trust to not buy stuff on your account... I'd never share my Amazon Prime Video access for that reason).
In terms of conversion avenue, consider e.g. kids moving out of the house. My son has his own Netflix profile. For now there's no incentive for him to get his own, and having him get his own would be impractical because he most often wants to watch it on my TV. But once he has a family of his own running into a profile limit would be an incentive to get his own subscription eventually.
A lot of people are in situations where they initially legitimately are in the same household, then move out for all kinds of reasons (flat shares, kids moving out, breakups), might not want the hassle of switching profiles right away, but eventually want more flexibility (more profiles), or privacy.
(Incidentally, a feature they really need is a "convert this profile to a separate subscription"; that goes for a lot of services that are not at all geared for a situation of kids growing up and wanting to untangle their online lives from that of their parents)
A lot of this problem would also just go away if they priced based on a combination of simultaneous streams and number of profiles and stopped worrying about where they are. E.g. there's never any reason for more than 5 simultaneous streams from my Netflix subscription. But if I had to pay per simultaneous stream I'd probably only want to pay for 3.