I think for many/most of them the key thing is that they need some 20x the amount of glucose than normally oxygenated cells. So they're actually supremely ineff

I think for many/most of them the key thing is that they need some 20x the amount of glucose than normally oxygenated cells. So they're actually supremely inefficient in terms of energy. What's the problem then? They just won't die off normally via programmed-cell-death (apoptosis) or autophagy or immune system attack, but keep replicating. Blood sugar levels may modulate rate of growth and metastasis, but since blood sugar can never go too low, that's only part of the picture.