So the data nerd in me is struggling with your framing a bit. HIIT for people with "aerobically poor systems" may resemble slow jogging or fast walking based on HR alone. Also outside of analyzing blood serum or glucose monitors, there isn't much of a reliable way to secondarily analyze the lactate threshold (actually). There are metabolite analysis that can be done, or things like creatine supplementation to facilitate the "smooth" transfer between aerobic and anaerobic processes, but on a fundamental level, my understanding as a lay person, is aerobic exercise produces lactic acid as a byproduct, and anaerobic exercise uses lactic acid as a fuel source. That is the reason anaerobic capacity decreases, as your lactate threshold increases. The real reason you are subject to a much greater degree of boom-bust is "lactate threshold training" is about increasing the physiological load the body is capable of sustained aerobic capacity for exercise (which is what you're saying), primarily by training your body to increase the amount of available glycogen. That's really only useful in the context of running and a few other endurance sports, and is hardly a metric of overall health.

Also IIRC the story of Marathon was relevant because the runner died to deliver the message. That sort of implies, despite the feat, that not everyone should be able to do that, nor should they.

HIIT conceptually depends on what your body does in a given state, not a unit of exercise. I would wager large sums of money the people that are overeager to engage with higher and higher intensities are not actually looking or reading the metrics of their own bodies, and adapting the behavior accordingly. It's not really about want or "determination" lol, it's about what you do to get your heart rate in a given place, and the fluctuations that produce an effect, given the systems in your body.

You kinda strawman these "HIIT people", as if they're actually doing HIIT, or as if they represent everyone. Autonomic dysfunction, based on overtraining syndrome or overexertion is not the same thing as observing a HIIT protocol based on your own biometrics. It's honestly hilarious, because HIIT and interval training at large is specifically designed to avoid OTS, especially where it relates to the serious damage that can occur to the vascular system, musculature, and ligaments, once scaling up the duration with a similar level of intensity occurs. Running a 5km in less than 20 minutes is a perfect example of a not very good metric actually. There are plenty of people that could put themselves at risk of damaging themselves to do such a jaunt. It is a very light entry point, but arguably is a good metric for elaborating on why training is important to be able to accomplish physical feats safely, but there's kind of a packaged Ableism in your argument that I kind of find distasteful. If your doctor looks at blood serum, analyze heart rates under stress loads, and examines blood oxygen under stress and during sleep, you can pretty much guarantee they will arrive at some degree of analysis that could prove (outside exception of health conditions or illness) a degree of general health and fitness.

I understand running is important to you, personally and culturally. Don't attack tools in the toolbox of fitness, just because you see other people using them wrong (edit: you did say you do use it). Everything in the world is a nail if you're holding a hammer, etc etc.

Conceptually, most of what you said is sound, but yeah. I think you're a nerd for process, which is cool, making kind of misguided arguments that are not really about what we're talking about here, which is why the article observes a principle experimentally we've known works for over a century. Also I personally like HIIT, interval training, and HIRT especially for all of the reasons I've described. :)