I used to think like you, but the fact of the matter is that most animals simply don't get cavities. Seriously! I mean, their teeth are very capable of getting cavities, but haven't you wondered why humans have teeth that "go bad" without regular maintenance? Have you ever known a dog to floss? The idea that all calories are equal is a tempting one, especially to engineer-types like myself, but it doesn't seem to be true.
Eating simple sugars is quite rare in the animal world, and presumably primitive humans. We like them so much because they are simple, high-density sources of energy compared to extracting a few calories from some nuts and greens. An early human would get as much as they could - which wasn't very much at all. But we are not set up to mostly run on them to the extent that we try today, and I think the evidence on that is increasingly clear. It's not necessarily simply a question of physical fitness, though it's true that that will probably mitigate many of the downsides like weight gain. The input matters, and calories and nutrients are not necessarily fungible - it doesn't go without saying that getting all your calories and nutrients via soda and multivitamins (and I guess fiber pills) is equivalent to e.g. a balanced diet of vegetables and protein even if the caloric and vitamin content is exactly the same. This of course ignores the fact that it is far, far easier to blow through your calorie budget with high-density foodstuffs.
It is a hard problem to solve. The basic problem is that our favorite foods bear no relation to foods that we should be eating, which was fine when the only foods there were to eat (mostly) *were* the foods we should be eating - or vice versa, the foods that are good for us are the ones that we evolved to eat. But we have an artificial abundance of the foods we really like, but didn't used to be able to get in common practice.
And for the record, I eat like a pig, am overweight, etc (though I'm working on it now that I have time). I do not practice the "paleo" fad diet and think most of its claims are bogus. But even though we don't know much about what "primitive" people actually ate, we do know that simple sugars are rare in nature unless artificially grown. Humans are clearly quite adaptable when it comes to diet... but perhaps not infinitely adaptable. We already know that trans fats are shockingly bad, for instance. Perhaps this applies to simple sugar as well - both are found in nature, although much much more rarely than we have been using them. If for no reason other than calories, most people would be better off eating no sugar at all - which would make it much harder to have stupendously high calorie diets.