Going by that article from David Gaider it seems that if you referred to a modern qunari as "kossith", you'd generally just get blank stares as most of them don't even know the term.
But the idea of having words other than "qunari" to refer to qunari race even if that's not the word the qunari themselves might use... doesn't feel like something objectionable to me -- not when people bring up example of Jewish people and it doesn't even register they're using English term instead of the actual Hebrew word that's quite different. And odds are pretty good that similarly the qunari word for say, "elf" isn't exactly "elf", etc. I think they'd be reasonable enough to recognize that.
People have often used "Jewish" as an example before, but I prefer to look at what we're trying to define and try to work out how to take it to it's logical extreme, then see if it still holds up?
Using Kossith to describe the grey skinned race currently known as Qunari doesn't work, since Kossith was the word for their former society or nation, something that no longer exists in any meaningful way.
For a good analogy, let's use the archaic term "Aengli", used to describe some of the tribes living in post-Roman Britain and let's see if could make sense to describe a modern Englishman. I mean, the word means "English" and the country is still called England, so what's the fuss about using it?
Well, let us suppose we have a hypothetical Englishman, who was born in England and who self-identifies as English, but who's grandparents arrived in England from Jamaica in the 1970s? While this person is obviously English without question, could you then still use the word "Aengli" for them and have it make sense?
Well, no. You're trying to argue that we can use an historical term for an Anglo-Saxon tribesman living in post-Roman England to describe an Englishman of Afro-Carribbean descent; two groups that are historically, geographically and ethnically unrelated to each other in an incredibly obvious way?
And even if we were to take someone who (improbably) could claim direct descent from the "Aengli" people, they still couldn't be accurately described as "Aengli" either, because they'd still be seperated by the gulf of over a thousand years from a way of life that they simply have no knowledge of?
This is essentially no different than trying to use Kossith to describe a modern grey-skinned member of the race now known as Qunari. The word, even if it may have been accurate in the past, simply has no meaning or relevance to the horned race anymore because the Kossith society, traditions and people have been lost to the passage of time.
If by some small chance we ever discovered there were actually some remnants of that society still living apart from the rest of their race and still practising the old ways (much like the Chasind and Avvars are compared to the modern day Fereldans), then these people would indeed be Kossith. But aside from that, the word should pretty much be consigned to the history books where it belongs.