Google threw the Android user base for a loop on Thursday when it announced in an email sent to GSuite administrators that starting on May 22, the Hangouts app would no longer be supporting carrier SMS. The app will still allow Google Voice users to send and receive SMS messages, but in the long run, this appears to be a direct follow-up to the reveal earlier in the month that Hangouts is going to pivot towards being a Slack competitor. The writing had seemingly been on the wall for a while, though: This move comes 14 months after Google started encouraging Hangouts users to switch to Android Messages for SMS and eight months after Hangouts removed its support for merged SMS and Hangouts chats with the same person.
In light of this news, what's a Hangouts devotee to do? Well, you have a few obvious options (not even counting the one that you may have from your OEM):
As noted, there's Google's Android Messages (formerly Google Messenger). It has the most similar design language to Hangouts, although it does make the curious choice to theme a given thread based on the dominant color in the contact's avatar. In spite of that key difference, though, if you like the look of Hangouts, you won't really get any closer than this.
Facebook's Messenger is an app you probably have already, and you can seamlessly integrate SMS into it. Like the current iteration of Hangouts, it doesn't merge threads between the same contact via both protocols, but for most people, this may actually be the best option. Why? It's probably on your phone already, and you can just drag it to your dock where you probably have Hangouts now and move Hangouts to the spot on your home screen occupied by Messenger.
Signal Private Messenger is best known for its own encrypted messaging service, but you can also use it for SMS. Like Messenger, it may benefit from the "already on your phone" factor.
Chomp SMS is notable for having the same default color scheme as Hangouts with the same basic design as well... but it's also heavily customizable. If the design of the app is what really matters to you, Chomp SMS is perfect.