Janet Rosen

{devil's advocate hat on}
Does it have to manifest exactly as it does in Japan for us to appropriate the words as something a little different from junior/senior?
Where I train the dojo culture has a several decades long history of using the terms to describe a chain of support and responsibility. I think that when newbies are told that is what "sempai/kohei" means, it has meaning beyond the English words "junior/senior" which in most contexts bear no inherent relationship. So while we are not using the Japanese terms to describe the full relationship as it exists in Japan, it becomes a useful jargon, if you will, so students understand it as something special within the dojo context, just as we use other Japanese language terms.
YMMV
{devil's advocate hat off}