If one wants to continue using Hangouts, then you would have to use whatever is required. I've seen nothing that says GV and the app will not work in the future. But with Google one never knows, and they could choose to end GV at any time. Google's hangman has been busy over the years, as can be seen in the following cemetery.
The Killed by Google list is indeed impressive. Among surprises to me was Google Site Search (used to provide google search function within an entities website), which I imagine caused significant upset to those using it.
The list touches on the edge of my most missed "killed" item-- Google Search, limited to blogs, forums, and newsgroups. (The list mentions Google Blog Search.). For me, the perfect way to research an item without having to wade through endless reseller sites, and links to reseller sites, etc. I used it multiple times a week. Unfortunately, it apparently got killed because it was not nearly as conducive to monetizing.......
So my confusion is. If Google voice app is essentially a better Hangout s. Like app.
Why would Google bring out new transition apps?
Google voice cannot do video calls or video conference calls, and so it's not essentially a better Hangouts app. Google has had a problem making an all encompassing app for messaging, calls, video calls and group chats, etc., and so it now takes a one size does not fit all approach and has several apps that each do at least one thing well. The following link is probably a good summation of Google's communication apps.