A question on evolution?

The earliest humans did not have any sort of dental care whatsoever but their teeth were integral to their survival and hence a huge factor in the evolution of man. In fact we could say that since they were carnivores that having functional teeth was certainly an evolutionary advantage. So, their teeth must have lasted a lifetime, albeit maybe only 20-30 years back then, but that would have been 20-30 years of NO dental care whatsoever. How did their teeth last? I'm 40 and have been taking care of my teeth all my life with the most modern products and dental care available and I have a head full of cavities, crowns and the like. I ask this because it would seem that naturally healthy teeth back then would have been an evolutionary advantage. Again: 20-30 years of NO DENTAL care. Can you imagine what someone's teeth would be like today if they did that? How is it that our teeth are so fragile now? It seems that if they were good enough millions of years ago to last 20-30 years with no dental care, how come they don't last 60-70 now with the sort of modern dental care we have?
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Firstly, we have a LOT more sugar in our diet now than prehsitoric man did. In fact, our cravings for salty, sugary, and fatty foods are (or were) an evolutionary advantage, because we got so little of them, that it was best that we scoff the lot when we find them. (obviously, this is backfiring now that we have a plentiful supply of them) Secondly, as long as your teeth will last until you have reached reproductive age and had children, then they are evolutionarily successful, as you have passed your genes on. From this point of view, evolution doesn't "care" how healthy you are beyond that stage. So if prehistoric man's teeth could last until he/she was say 20 years old, then they're doing just fine, and there is little selective advantage to having them persist beyond that stage.
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