Google won't kill Hangouts because it thinks the two serve different purposes.
For one, Hangouts is tied to Google's enterprise For Work products that's aimed at business customers, and which includes Gmail and Docs, and is available on desktop. Allo, by contrast, is mobile only, doesn't require a [Google account], and is focused on the power of artificial intelligence.
They could make a web client for Allo, just like WhatsApp did. The Google for Work thing seems to me to be the singular reason to keep the Instant Messaging part of Hangouts intact, and is that really enough reason, if consumers move over to Allo?
+Abhilash Bingi Blogger is getting updates . There haven't been anything sexy like new templates, but there have been lots of updates behind the scenes (most recently implementing HTTPS). So maybe don't expect anything too shiny, but core functionality is continually improved.
If Allo doesn't have a webclient I certainly won't push my friend's and family to adopt it. When Google added SMS and finally Google Voice SMS to hangouts I thought they were really going to try and make an iChat competitor (service that sends the right type of message depending on where the recipient is connected.) But no... they've made Hangouts SMS worse and less integrated and now Allo looks to be an incomplete product.
Remember when Google Talk (hangouts before the hangouts) federated with AOL Instant Messenger? It's like 1 step forward, giant leap backwards with IM all the time.
+Paul Klapperich having played with Duo, the power of the app is that it's stripped down and does one thing - 1:1 video calling with phone contacts. I suspect that Allo will similarly be powerful simply because it has limited features. For those of us who want full-featured experience, Hangouts is the app/web client to use.