Yeah... I am not sure what Google is up to with its messaging platforms. Near as I can tell, we're smack dab in the middle of a big digital soup... First, they've already announced that Hangouts is going to move to a more Slack-like model. It'll be a platform for group communications and collaborations and not as much a messaging app.
Allo is the personal messaging client of the future... but its not fully baked at the moment. It still needs to have a few things.. notably the ability to handle SMS/MMS natively, allow for multi-presence, and store messages in the google cloud so conversations are maintained.
Google Messenger, well, that'll always stick around I think, as its the Google build of the AOSP SMS app. Allo is not an Android core app, so there will always be a non-Allo SMS client out there.