Install the Galaxy Nexus Messaging app onto the Nexus 5
You're not alone in wanting the old Messaging app!

Even though the old stock Messaging app is no longer included in Android, you always have the option of installing an alternative. In fact, people are recommending 8sms as a replacement for Hangouts that is almost identical to the original stock Messaging app. In fact, it is built from the source code of the original Messaging app, with "some enhancements".