A difficult subject and tied up with so many considerations around human relationships.
In my personal view, I think you are painting the picture a little too negatively.
When a chief instructor promotes a student toward shodan or beyond, it is with the clear expectation that teaching and administrative burdens will be distributed, and the ability of the dojo to better serve the larger community will be enhanced. Each student that simply disappears is an investment gone bad, no matter how regularly they paid their dues.
I have seen plenty of situations where students do take on more responsibilities and things work very well (at least for a time). And yet I have also seen promotions be used almost in a sense of trying to put obligations on to the recipient, and as a form of emotional blackmail. There can also be a subtle form of boosting one's own importance by having more senior students.
Maybe I am misinterpreting your point about "investment gone bad" but I disagree strongly
We all have interactions with other people all the time, and thus typically have some sort of effect on those people. This is often not a lasting effect, but it certainly can be. Within a dojo setting the interactions tend to be deeper, particularly where there is student/teacher relationship present. So if someone leaves a dojo then they have still been altered by what took place within that dojo.
I have visited dojos and had relationships with teachers that have had a lasting effect on me, even though I have subsequently chosen not to study further there (sometimes for practical and sometimes for other reasons). I feel a loyalty to a something larger than just individual teachers (articulating precisely what this is is one of my goals!), even though I do feel that too. Sometimes loyalty to the individual is not enough and I have moved on.
The teachers I most respect (and use as my models) are remarkably clear and consistent in what they do. They are also very generous with their time and energy and what they pour in to their students. They also don't seem to have their own ego bound up in what their students do or do not do. Two rather different examples are Suganuma sensei and Inaba sensei. Suganuma sensei has built up an organisation consisting of around 100 dojos in and around Fukuoka in Japan and I find his whole attitude and approach very nurturing and supportive (there is a lovely pictorial bio on http://www.geocities.com/Tokyo/Dojo/8846/biopage.htm)
. Inaba sensei is different - a relatively small direct organisation at the Shiseikan, but with growing influence overseas. His teaching method is quite personal and direct, the major influcence being his study of the koryu Kashima Shinryu with Kunii Zenya. And yet I have seen Inaba sensei pouring his energy into visitors and also university students who typically only train for a couple of years before giving up budo when they get a job. He thinks for the long term, and he knows he has had an effect on them that will continue, even if he never sees them again.
I will stop there for now - interested in your thoughts?