I appreciate you sharing your experiences, but I think you've missed a whole segment of the population. I can't provide any sort of scientific analysis but I can tell you with some certainty that there's a whole class of people - doctors, lawyers, airline pilots, accountants, police officers, firemen- for whom it's become a cultural thing. These people have organized clubs and social groups, posted to Facebook pages and created web sites.
Nobody takes it too seriously but at the same time they're doing 2 or three halfs (halves?) per year. My professional association sponsors one each spring plus a 5K in early summer. Many of these people would buy a photo each and every time if it was only ten bucks- it's a little more personal than the medal or the teeshirt. Being in the group photo to be posted in the corporate magazine has become de rigueur. Unfortunately, in our case these company trophy shots are still being taken with iphones and pocket cameras.
Nope. Have not missed your segment. I think you for one over estimate it, i.e. people really quickly fall into the bucketlisters or the enthusiast and the excitement of photos, especially PAYING for photos quickly wears off even at $10 a pop. You also over underestimate the time ans expenses to do decent shots, go through and select, process, tag, and make available. A few orgs I know that are out there GIVE their photos away... i.e. they pay the photogs just like they would pay Brightroom, but instead of having sales, they give the photos, just like the swag T-shirt or medal, etc.
Even if such a segment may be a little larger than I think, it is still vastly smaller that those in the groups I mentioned. You have 50 people come into your area at the same time, and you only have a few seconds to acquire, frame, focus and repeat. Triathlons as well tend to spread out a lot more than your average road race, so much easier to shoot the bike or the run portion.
Long and short, you have a small window to pick and choose your targets, spend 3 seconds trying to wait for the target to put on a decent face or another runner gets in the way and you have lost the opportunity to shoot another 1 or 2 runners that you will not get.
It is EASY to sit there, pick out a dozen people you want to shoot at a race, shoot them, and maybe even shoot them in multiple places or from the same place in say and out-n-back portion of a race.
It is a far different beast to have a race with a thousand people and try and shoot 950 of them to try and find the ones who MIGHT be one of the smaller segment that might want to buy photos. There will be people who are always blocked by someone else and you never get a clear shot, their face or set up looks crappy, and taking a bad picture is worse than none at all, and actually as bad as some of race photography is, a lot is discarded because of how embarrassing it is, and lastly you even have a dozen people come through, you pretty much have to pick them off as best you can and half the time after you shot someone and moved on, THEN they give you a better look that you snapped.
Forgot to mention. Most of your shooting locations, there is a small "sweet spot" where the background is appealing, you don't have a mail box, road sign, telephone pole or other crap in the way. There is also a sweet spot of angle shooting the runner or cyclist that looks good to blah, as well as once a person starts getting at an angle to you, you loose their bib number or the bibs drift to the other side of their hip, etc.
My suggestion... Find a large race... sit out there and try and shoot. Granted a lot of event photography is crap because the shooters are not athletes, and my photography in this milleu is better because I have a relationship with the athletes; I shoot them as I want to be shot as well as from feedback of what people really like. I think what you will find is that your photography may be a better than the event photography, the majority of the images drift down the scale as you start shooting volume compared to when you are able to focus on a dozen or so athletes.