RE: "Martial arts may have been created for the explicit primary purpose of conferring combat ability on the trainee, but it does not follow that they continue to exist for that purpose.". No, in fact it does indeed follow that they still exist for that very same purpose by definition. If they still continue to call themselves "martial arts", then that is the purpose they serve. If a practice is found to be no longer viable for its intended purpose and serves no other real purpose, it should be acknowledged as obsolete and discarded, or at most, re-labelled and marginalized to a historical/cultural study only. If a practice is found to be no longer viable for its intended purpose but still yields desirable benefits of another kind, it needs to change its name/label/claim/advertising/etc. in order to reflect that newer, more accurate reality.
RE: "Of course one could argue that by it's definition a martial art must relate to martial usage. Therefore, if practicing bagua cures cancer but does not make one a better fighter, then by definition bagua is not a martial art, regardless of how useful and wondrous it may be-- which is why I said that argument requires a very narrow definition of martial art (and a rather limited definition of "use" as well).". That is exactly accurate, if one is practicing any degree of intellectual honesty at all. There is nothing remotely "narrow" about a thing being simply what it is by definition. It is far, far more accurate to say that if we wish to call something a martial art that bestows no appreciable combat ability but yields a host of incidental and serendipitous other benefits, we are forced to use a dishonestly broad definition of what a martial art is.
RE: "If we want to be pedantic, we can easily argue that virtually none of what is practiced as martial art (including RBSD) is martial at all, the meaning of martial being "of, relating to, or suited for war or a warrior", given the state of modern warfare.". There is no need to engage in pedantry either way. However, the arguement you submit here is not so easily argued, especially if you include RBSD. Granted, most of what masquerades under the term "martial arts" these days is woefully inadequate to the task, but that material which genuinely imparts personal combative ability is very much related to and suited for combative use, whether that use is in a war or for civilian application. Further granted, such skills are by no means sufficient for wartime purposes, but for certain personnel, they are most definitely still essential.
If there is a legitimate negative criticism of that martial art training (whether called RBSD or not) which does yield real skill, it's not that the skill is unrealistic in and of itself or no longer relevant, but that it is, perhaps even dishonestly, insufficient for the task in the modern era, whether in war or in one's own neighborhood. Times and technology have changed profoundly in the last hundred years when compared with the last several thousand. Firearms are not only technologically so advanced that they negate even the most advanced unarmed martial art skill ever developed by anyone, they are also so commonplace that it is becoming more and more naive to assume that a firearm won't be present in several types of modern assaults.
If we are truly intent on re-matching the verbage/claims/labels/etc. of what we do with true sufficiency to the actuality of it in today's world, to a degree that might even have once existed in the distant past history of martial arts as a whole, we have no choice but to expand the overall skill set dramatically to include the useage of and defense against modern weaponry, communications technology, computers, vehicles, and probably a number of other equally relevant things I'm forgetting to include. Speaking very personally, I've been there for a number of years. The glaring absence of such things in training is part of why I'm not personally comfortable calling myself a martial artist anymore. I made that statement on the old EF about 4 or 5 years ago, realizing that it was mostly semantic to everyone else but me, but it was something I felt I needed to do in the interests of intellectual honesty.
Will the culture and practice of martial arts as a whole evolve to reflect a more modern reality? Only time will tell. If it does happen, it will already have occurred later than it should have. And probably, like most such large-scale memetic evolutions, it will be ignored, ridiculed, persecuted and finally accepted as self-evident.