It's a bit odd that you are writing this while citing a blog that said that removing ConsoleKit (non-systemd) code was not done in Gnome 3.16 ?
But let's accept that for the sake of the argument.
It looks like GNOME wants to depend on systemd's public and stable DBus interfaces; this is good because it gets GNOME out of having to maintain OS portability gunk, and puts the burden of maintaining said gunk on other projects, i.e., the systemd project on GNU/Linux.
The goal is rather similar to what the OpenBSD community is doing in its applications, in e.g. the OpenSSH project, which (in its upstream release) only runs on OpenBSD, making maintenance easier.
There is a difference though between the two approaches: while OpenSSH has a downstream project OpenSSH-portable that patches the source code to replace OpenBSD specific calls with portable calls or #ifdefs, the GNOME strategy is to depend on abstract APIs that are implemented by some lower layer in the OS.