apt - Why doesn't adding the Kubuntu backports ppa allow me to get the latest KDE version?

I'm running Kubuntu 16.04 64-bit with:
KDE Plasma Version: 5.5.5;
Qt Version: 5.5.1;
KDE Frameworks: 5.18.0.

I attempted to get KDE 5.7 by running the following commands:

sudo add-apt-repository ppa:kubuntu-ppa/backports; sudo apt update && sudo apt full-upgrade

Result:

Reading package lists... Done

Building dependency tree

Reading state information... Done

Calculating upgrade... Done

0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

This is because KDE 5.7 hasn't been backported to Kubuntu and added to the backports ppa yet.

I've been waiting too, so I hope someone involved in the project will comment on when they expect that to be done.

Edit: I notice that you actually on KDE 5.5.5. You should at least get upgraded to 5.6.5, which is available for Kubuntu 16.04. Try running kinfocenter to double check your version information.

Update: It doesn't like like KDE Plasma 5.7 will be available for 16.04 at all, I think you will have to upgrade to 16.10 next month or switch to the Neon distribution.