My understanding is we look for calcium buildup in arteries because calcium provides a hard, reflective layer that shows up in scans, not because it poses the danger of blockage that plaque does. The calcium is an indirect measure of risk because it's correlated with coronary disease, not because it's the cause of heart attacks like blockage from plaque is.
The trouble with plaque is it's hard to detect without an expensive MRI or angioplasty. Studies show that when you change someone's diet who has calcium and plaque buildup, the plaque recedes along with their risk of a heart attack, but the calcium rarely does.
Retired athletes who have heart attacks are very common even when they cut back on their exercise as they age. For example, the great marathoner Alberto Salazar was only 48 when he had his massive heart attack and he had remained slender and kept jogging 25-30 miles/week. As in almost every case I'm aware of, he had no structural damage to his heart from his years of being a champion marathoner and ultrarunner. He simply had plaque build-up because he enjoys MacDonald's.
It's true, former elite athletes are more at risk of heart attacks as they age, but so is the rest of the population. Plaque buildup is cumulative over the years.
There are people with major structural damage to their hearts from past heart attacks or infections (that's me, Rheumatic fever as a child). They far exceed any damage a pro athlete sees to their hearts. But as the Cleveland Clinic Prevention Center has so dramatically demonstrated, it's a rare patient (less than 1 in 100) who experiences heart disease after a change in diet, even if they are sedentary or old. Their prevention center sees the worst of the worst, patients who've had stents, heart attacks, strokes, and bypass operations. Their surgeons referred them to the prevention center because they had become so weak or old that surgical interventions are no longer an option.
The main artifact from heart attacks and infections that a change in diet won't help is permanent damage to the nerves that carry electrical signals, but that only represents a small percentage of patients whose heart problems arise from plaque buildup.