I dunno, Google is sort of pissing me off, come out with new stuff, pump it for 6 months, then ignore it.
Does the world really need another messaging app? Oops, sorry, does the world really need 2 more messaging apps?
You say that like they are all the exact same messaging apps. Are they?
Google Messenger is an SMS client, that's all it does. Some people just want/need an SMS app, this takes care of that. They don't want a bunch of extra features/bloat that is useless to them, they just want SMS.
Hangouts is based on your Google account. This may be shocking to some but not everyone wants or has a Google Account.
Allo is an app similar to Whatsapp, tied to your phone number. This type of messaging app is always going to be the most popular, so I understand Google going after this market. There are a LOT of Android users in the world, but not all of them have Google accounts. Google wants to reach these users, to get them to use the built in search which can get them to use more Google services.
Duo is the video chat app. Knock Knock is the single best feature I've ever seen with any video chat app, can't wait to use it. I believe in the presentation they said that Allo would use Duo for its video chatting?