Google pulling the rug out from under Voice. What will remain?

Google wants their Hangouts platform to be the only solution people need. They are competing with iMessage (combined chat and SMS), Skype and Facetime (video chat) already, and the integration of Voice will allow them to compete with Skype, Facebook Messenger, and other VOIP apps (on web and mobile). In order to fully dominate, they will need to increase platform features rather than eliminate - so I can't imagine them killing anything you need here. The only reason they killed off XMPP support is because, despite their propensity for open-source solutions, all their competitors are using closed systems. By designing and integrating their own closed system they can develop and introduce new features much more quickly.

I don't think anyone besides people behind the scenes at Google knows specifically what the extent of the Google Voice + Hangouts integration will be. If you're open to other providers for telephony and unification, I've heard very good things about https://www.line2.com/ from a coworker of mine.

I couldn't find any official announcement or even find where you got the May 15 date from. In any case, I think if you get a Skype number you can use the Skype app (either on iPhone or Mac) to receive calls, text, and check voicemails. I'm not sure however how their forwarding works, if it's sequential (rings on skype first, then somewhere else if no answer) or in parallel like Google Voice does. I didn't quite understand this line of your question though, could you clarify? "Everything I'm hearing about seems to be focused on VOIP calls but don't mention the existing functionality of Voice." Do you mean the voicemail transcription? Forwarding? Texting? The reason why you don't hear about the other functionality is that with most serious VOIP providers (i.e. not bundled from cable) offer most of that. Though the transcription is actually rarer in my experience.

i was assuming May 15th was also the date for Voice and Hangouts merging. It seems that is happening at some point later this year. The functionality I need as I said above is to use the service with both an iPhone and land line. Line 2 mentioned above seems to not work with land lines.

I'm not familiar with the specifics of Line2 service, but looking at this page: http://support.line2.com/entries/21359568-How-Do-I-use-Line2-from-a-Landline-Dial-in-Access- seems to imply that you can set landlines as forwarding numbers (to receive calls.) The instructions for dialing out from a landline using the Line2 number in your outgoing caller ID are on there too; once you have the landline set as a forwarding number, calling your Line2 number should give you the option to make an outgoing call to your client / destination. That sort of thing could probably be automated with a speed dial sequence you could set up on your landline phone, so you'd pick up, hit the speed dial button, then wait for a second dial tone after hooking up with Line2, then be able to dial out.

While I can't link to a specific announcement, I am 99% positive that the 3 features you list will continue to work in Voice for a long time, probably forever (regardless of Hangouts integration). And May 15 is just for the XMPP stuff, not for removing the Voice app or anything. If you're not using VOIP nothing will change.