Hello veejay.
Thanks for your informative post - I certainly intend to look into having a colonoscopy as soon as I can be sure that the instrument can be pushed in without causing any damage.
As for people who don't get actively involved on this site, there have been posts on this subject before and we can only guess at what the possible answers are because if they don't reply and tell us there is no way of knowing. However, that shouldn't stop us from speculating so here goes:
From my perspective regarding my own posts (which are mostly rhyming verses). I think many people simply read them and do not consider they need to reply. Of course I am very grateful to the folks who do comment but I also understand that there is really no need to reply to this type of post. I often think that it would be useful to have one of those tick-boxes that indicate if people like the post or not, so that they don't have to find words to express any more than that. Further to that, most of my initial posts are on my blog and there is a system of counting how many people have had a look at each one. This gives some idea of the percentage of people who make comments , compared to the number of people who passively read the posts (usually very few).
As for people who post and then don't follow through, there could be a number of reasons for this. I do not think we can assume that they don't read the replies because we have no way of knowing this one way or another. If they don't reply to the responses, it may be that they have got the information they need and simply go away satisfied/ or they did not find satisfaction and gave up on the post and/or the site for that reason. There have been a few people who have replied in this vein in the past so we may assume that there might be others with similar feelings.
Alternatively, there are some people who post on here simply to vent the feelings or thoughts they have at the time and once they have experienced the cathartic effect of venting, they really have no further need to stay involved.
My overview of this subject is that a site like this is probably a reflective microcosm of society as a whole inasmuch as most people are on the sidelines, looking in as passive observers/spectators, rather than being really active participants. Every now and then they might contribute a word from the touchline but that is usually the extent of their overt activity.
You mention 'common' courtesy, which is a bit of a misnomer in the modern age. It seems to me that it is much more 'common' for people to have little or no courtesy at all nowadays. The idea of 'common-courtesy' is of a bygone age and represents a concept where it was hoped that people would have consideration for others and act accordingly. Unfortunately, there is a growing tendency for people to be self-centred and selfish rather than considerate so being courteous to others does not enter their consciousness.
Fortunately, there are still many folks who do still value a less self-centred approach to living their lives and we can only hope that this sets a good example to those of a different persuasion.
There are many other things that could be said about our society and its values that are tangential to this discussion but are really central to the whole study of sociology in a modern society. This does not seem the place for such an in-depth discussion on the subject so I'll stop here.
Best wishes
Bill