de0u However, though this is admittedly speculation on my part, given the existence of short-term "teaser rates" (time-limited discounts for new subscribers), it seems at least a little bit plausible that a secondary desire of streaming services is, indeed, identifying individual devices over time. If that weren't the case, people could reset to the cheaper teaser rate repeatedly by forgetting the Widevine keying information.
I haven't seen any streaming service offering such discounts while also allowing you to pay anonymously. Usually when paid streaming services offers a "first month free" discount or similar, they still require you to sign up for monthly charges by registering your debit card and then approving a zero charge for the first month. So in that case the persistent identifier would be your real identity instead. I guess tracking persistent identifiers on devices would be seen as an alternative to be able to offer such discounts while allowing anonymous payments, it would still be more privacy respecting, but it doesn't seem to be anything they care about today. And ad-supported as well as free streaming services obviously have no discounts, so wouldn't need to know any persistent identifier at all, just that you are in the right country and that you cannot copy the video stream.
I have been able to take advantage of discounts, while streaming solely inside a pure virtual machine. So they definitely aren't checking for persistent hardware identifiers. Quality may be restricted to 480p or 720p though when not having hardware based DRM, even when paying full price. But for discounts they seem to rely on other things, as I have been able to take advantage of them anyway.
But maybe it might make sense, to have such a persistent identifier, if we wish for them to start accepting anonymous payments in the future.