All of them? Certainly not. Many? Yes.

>> There might be some friend groups who split the cost of an account between them, but I suspect far more common is that parents have an account that kids (who might be independent adults) use, couples who don’t live together sharing passwords

Yes, but although we don't have details on demographics, in Europe it is widely "accepted" (for lack a better word) that many friends, relative and even acquaintances share an account and its costs. I don't have number so consider it totally anecdotical, but it is honestly common sense, especially in southern and eastern Europe. (I am from Italy, FWIW)

edit: this is also true for Spotify Family accounts. Agains TOS? Sure. Do they do it anyway? Yes.