It also gives you 4 simultaneous users with the premium plan, which I would guess most families use simply based on that.

If they’re sharing logins, maybe. I bet most families that manage to stream four things at once almost never try to do so on a single service at the same time. Can’t you still get Disney+ for about the price of upgrading to 4K Netflix? Or some-ads Hulu.

Of course, that's the point of the account to share logins, you still get your own user with your own lists, recommendations and all that. Though, Spotify handles it nicer where you can add your own account to the family group plan.

Not often, but often enough to become a nuisance with 2 TVs or so, some tablet/laptop for school work/gaming/whatever and a mobile device per person.

Yeah, I think that's about right.