Excellent question, Iceman.
As serious, dedicated Martial Artists, and instructors, I think we have to bear in mind that some of our prospective students won't be looking to do much more than "try it out." We know that there are many benefits to training in the Martial Art beyond self defense, physical fitness, trophies, and the 1st Degree Black Belt, but not all new "potential" students are ready to commit to a life of being a Martial Artist.
Sean (Touch of Death) makes a good point that we can lose potential customers, who might eventually become dedicated, life-long Black Belts, if we scare them away too early with the "It's a lifetime of learning" philosophy. On the other hand, there are those who will walk away if they feel you do not grasp the longevity of commitment as a Martial Artist. So, your point is a good one, Iceman. We do need to make some mention of the long term goals.
I find it best to have a private appointment with any new, prospective students and find out what their focus is. I will make verbal note of the various commitments that each level offers, including the dedicated student for life concept, but if they seem less certain, then I would rather get them to join with more focus on the short term goals. In time, I will likely be able to teach them about what is beyond the 1st Dan.
"Are we saying enough about the art?" Perhaps not. I think we should make mention in our websites, and in our presentations that there is the "option" of a lifetime of learning in Taekwondo. However, we must be cautious that we do not say too much. Let them ask if they want to know more. I let people know of the benefits they can achieve on their path to the Black Belt, but I do not imply that it is the "end of the rainbow." I always define the Black Belt as a mastery of the basics, and the beginning of a further education, if they so choose. By the time one of my students reaches Black Belt, I have had plenty of time to convince them there is no reason to quit training - - ever!
Chief Master Eisenhart