As they just did to all the Nexus users, where last year's phone buyers won't get software-only features like the Google Assistant, just to encourage new phone purchases.
The new launcher is called the Pixel Launcher (though leaks of it called it the Nexus Launcher). Speculating that Nexus users won't be getting the new launcher is reasonable (though they can just install the APK and it works fine). That has nothing to do with the Google Assistant though.
There will be a developer preview of 7.1 for Nexus devices in the next couple of months. Having to wait a couple months to get the latest and greatest Android release after it's publically available on another phone is definitely something Nexus owners can complain about. There is nothing to suggest that Nexus phones won't be getting features like Google Assistant though.
http://mashable.com/2016/10/06/google-assistant-not-in-andro...