As for the Nexus 5, this smartphone went for 3 years, he was released in 2013, and Google no longer intends to support it. However, you should not even doubt that firmware with Android Nougat for Nexus 5 can be downloaded from the XDA forum.
we have all Known Evan Blass recently said on Twitter that soon the world will see the release of the final version of Android 7 Nougat. He says that Google has scheduled the release for August, with a fresh security patch. But, unfortunately, the owners of the Nexus 5 officially it will not get. The first would be owners of Nexus 6, and after smartphones of different vendors.