According to their Terms, there are no legitimate sharing use cases. "You agree that you won’t disclose your Account password to anyone". That said, Calm has a family plan (6 users) for only $30/year more.
To meet bare minimum requirements sites like Calm.com often only show the "Sign In with Apple" button for iOS and iPadOS user agents. You can sometimes fake an iOS/iPadOS user agent and get the buttons to show up and then switch the User Agent back to get the boring in-browser OIDC flow.
As an iOS/iPadOS but Windows PC user I find it frustrating how many websites intentionally hide that button in other browsers, just when dealing with my own accounts.
I feel like Apple could maybe do a better job of education here: "please don't hide the button on non-iOS/iPadOS user agents". I hope Apple has strong developer education plans for Passkey.
Anecdotally, in the instance you described they net the same $60/yr regardless of wether you use your wife's login, so they're getting more stringent access controls (though you could always use the device in question, too) in line with their terms of service.
(Would also note that my response does not cover your latter points on helping users manage their accounts with their presence, but without their device presence, which I agree is a valid concern)
Apple says you can "download content" bought by other family members[0], but I guess this doesn't work for subscriptions?
That's a real missed opportunity. I would expect them to at least allow the app developer to opt in to sharing. Selling two licenses into a household has to be pretty rare, whereas having happy-family customers is great word of mouth. "My wife uses FooApp and now I use it too!"
Family Sharing is set up in such a way that you're punished for creating child Apple accounts as Apple recommends, because you'll need to repurchase MOST IAPs several times over.
Apple's subscriptions are the exception rather than the rule, but they like to promote it in such a way as you'd think all Subscriptions/IAPs are shared when they're not.
But since this experience has been so confoundingly annoying, Calm.com won't get a penny from me. (Not even sure I want to blame them... but rather how it's impossible for my wife to just "share" that credential with me even if she wanted to)
Not being able to share a certain type of login (for this kind of family sharing scenario) has really soured me on the more personal use cases for these technologies, even though they are really solid from the tech side.
Like people often say, most products are competing with Word, Excel, and email, authentication is competing with me just being able to tell a human my username and password.
Again, there are many instances where being able to share these credentials seamlessly between non-power users is totally legit. When I sit with my dad to help her log in to his patient portal to get some medical info. Or when my mom wants help to review her Verizon cell phone bill. Or when I get my wife's credentials to cancel a hotel reservation.
How are they going to solve the totally legitimate sharing use cases? Using PKI actually gives you a wonderful set of tools to do this (underlying implementation could be that we generate a temp cert that expires in 24 hours, signed by her original credential certificate saying it's a shared credential, etc)
I really hope these are the use cases that get more fleshed out before these technologies reach mainstream.