I agree there is some ambiguity in calling it theft, mostly because of the requirement of the intent to permanently deprive the other of the thing, but I do think it broadly fits the definition based on the idea that the taker is permanently depriving the owner of the value of the IP even if they aren't actually taking a thing. But, as you point out, that's probably why they are writing more specific laws to deal with the issue.
Still, I do think it's fraud since you are essentially representing yourself as being the person with the subscription by using their username and password.
But, obviously no DA is going to ever prosecute this stuff anyway unless it is done by someone selling usernames and passwords in massive numbers.
Anyway, regardless of if it fits the legal definition of theft or not, I think we all know that it is stealing in the practical sense even if we choose to play semantics in our own minds to avoid admitting it.