These are all looking at various surrogate endpoints, and seem to be short term interventions. At best you could say they show saunas are safe for populations with heart disease. Cardiac and overall mortality would be the key endpoint to test in a large RCT. Even so, there won’t be a placebo. The control group would have to do something for it to be a fair comparison… perhaps something sauna-like but with less physiological impact (Massage? A cooler sauna?).