I admit I also like to keep an up to date dCore, which can be done with Ubuntu non-LTS releases. We can ponder it more, but I think the majority would rather focus on quality rather than quantity. And when we take into consideration the security updates of packages and backports for a certain dCore port, that is a another set or two of packages to maintain and troubleshoot, and those package versions change as they are kept up to date.
jls - I will mail you a link to a tarball that will contain the directory of scripts and root filesystem that the dCores are made from, there is one for each dCore, I will send dCore-yakkety. Everything is scripted, or I could not keep up with it all. It is pretty much personal scripts of basic commands to create the dCore-*.gz image out of a rootfs directory, the commands are mentioned in the Remastering part of the wiki.
End of life Ubuntu based dCores can be archived while their DEBINX files kept online as long as they are available from Debian/Ubuntu, for those that are older but based on actively maintained releases they can stay in the main release/RC area, ie dCore-wheezy.