I'm familiar with the common use, and even there I demur. He was a mature man who was using and adventure model segway to travel off-road. Death by misadventure may be statistically unlikely, but it is not unexpected. Furthermore, in his life he was able to accomplish a great deal, and while he may have gone on to accomplish even more, I trust that we can look at his life without a massive sense of what-might-have-been.
Compare and contrast to the "tragic" death of Jimmy Blanton, Duke Ellington's bassist for just two years. Blanton is credited (or blamed if you prefer) with inventing the Bass Solo, but at age 23 he died of tuberculosis. Thus, the world was deprived of the bulk of his career and looking at his short life one wonders what Jazz might have been like had he played for five decades like Ray Brown, a man he inspired.
The tragedy of Blanton's death was that it was significantly premature. Naturally the bereaved think that Mr. Heselden's death was also premature, but we must draw a line somewhere, otherwise every death is tragic and the word means nothing at all.
So Blaton's death was tragic in comparison because of how premature it was and how much of his productive genius was lost. That being said, the cause of Jimmy Blanton's death was not extraordinary given the health issues affecting many from his social class at the time.
This is different than Scott LaFaro, another pioneering Jazz Bassist who died "tragically," however Scott's death was in an automobile accident, an unexpected incident. Scott's career was cut short and it was cut short in an unexpected fashion, another mark of a "tragedy" in the sense you promote. Mr. Heselden's death was anything but unexpected. If a man sets out to seek adventure in wild places, he and everyone around him must accept the risk that he will not survive.
If I am to die while Scuba diving, could we really call that a tragedy? What caprice of fate can we blame for a man who is killed when he deliberately immerses himself in an environment hostile to air-breathing life?
This watered-down use of the word "tragic" no longer carries much meaning, if any. I get that it's the style (especially in America) to say that since English is a "living language," it is defined by whatever meaning the vox populi care to give it. But while I respect your point that there's more than one way to use the word, I feel that even this lesser meaning does not apply to a mature, accomplished businessman who died while knowingly engaging in a risky activity.