We don't call Windows by it's kernel. We don't call Mac Operating Systems by their kernels. I don't feel comfortable calling GNU/Linux by it's kernel. Not in 'formal' settings, at least. When I talk to my friends, we all understand what I'm talking about. I don't feel the need to say it every single time. When I'm introducing it to somebody, or writing material about it, I tend to use GNU/Linux.
Neither GNU nor Linux is a full system, and just the two together still isn't far from a full system, while the GNU project originally intended to develop a full system, a full system with nothing but software from the GNU project does not exist. Neither is Linux a full system, for it is just a kernel. Ubuntu, Mint, Debian, Fedora etc are distributions, i.e. full operating systems, so the full system should be called whatever it is called.
Linux has always been called Linux, I do not think it is necessary to make the title longer and more winded. I think even if it got changed to "GNU/Linux" people would still refer to it as just Linux. It will be annoying and longer to have to add the GNU with it.
I don't think that it's necessary to call the Linux operating system by the full GNU/Linux name. I think that everyone who uses Linux is well aware of what it is and what the underlying principles are, and the only point of requiring the longer name is for those who wouldn't care anyway.