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However, it could be argued that access to it may be removed or just stop working since Hangouts does NOT support the ability to call yourself (dial your own GV number) to access voicemail.
You keep theorizing and speculating based on incorrect assumptions. There is nothing whatsoever to be inferred about Google Voice support for OBi devices, based on the retirement of the Chat GUI.
I can't make this any clearer: separate the concepts of
Google applications or products from
protocols.
There are two control protocols, XMPP and WebRTC. There are several products that use the protocols. OBi devices use XMPP. Chat and Talk used XMPP. Hangouts uses WebRTC. WebRTC is specifically designed for peer-to-peer audio/video communications over web browsers, and it has commands that can access the computer's camera and microphone/speaker, and set up calls. Google has been systematically retiring (or "deprecating", to use the jargon) all of their own products that used XMPP. They've been doing this in stages, for over two years now. The last step in that timeline is to shut down the remaining Google products (the Chat and Talk apps) that used XMPP. Google is leaving the XMPP protocol in service, to support OBi devices. Hangouts has absolutely nothing whatsoever to do with any of this. Both control protocols use the same CODECs and can both use the same media transport over UDP. Hangouts adds additional, wideband CODEC support and support for transport over secure connections. This additional functionality is used by the Hangouts web clients, which are not limited by the narrowband audio standard of the PSTN.
Google Voice is primarily a PSTN call forwarding and message management product. By itself, it is not a VoIP application (at this time, you cannot make nor receive VoIP calls using Google Voice by itself).
Hangouts was originally intended as a web-based, peer-to-peer A/V app. As a nifty add-on feature, it was given the added capability of making PSTN phone calls via a VoIP gateway. It is not intended to fully duplicate Google Voice functionality.
Again: Google is killing
their own XMPP-based apps, but leaving the underlying protocol in place.
If a user has a computer, and can use Hangouts, then they can easily listen to their VM messages by simply playing them on the computer. GV will text and/or email a notification. The email notes have a link that takes you to the recorded VM message, playable in MP3 format. There is no reason whatsoever to use Hangouts on a computer to listen to VM messages.