We don’t know that it has stood for millennia in it present form, nor can you argue that, “...portal tombs with their extreme sloping capstones or massive capstones perched on three points and also sloping...” are any less stable than a horizontal capstone on four points. In fact, without any formal training in structural engineering, I'd hazard to say that the former is probably more stable than the latter, certainly easier to build and, to return to aesthetics, far more pleasing to the eye :-)
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