I do not claim that breast feeding is no better than formula. However, I am currently convinced that there is not yet sufficient evidence that breast feeding a child will likely lead to significantly better long-term outcomes than feeding formula.
I'm a scientist. Not a medical scientist, but a hard science non-the-less. MDs are not (usually) scientists. They go off of what they believe the scientists are saying. But it's a game of telephone and you're now the fourth in the game (data -> researcher -> MD that is your teacher -> you).
The studies I've seen (and I'm NOT an expert in the field or in the techniques used for these kinds of longitudinal, observational, social studies), mostly show very minor differences in long term outcomes. Honestly, the long term difference is probably much smaller than many other lifestyle choices you could make.
Some sibling studies apparently have shown no significant difference in outcomes. Some try to control for socioeconomic status (how many people ever ask how they control for these things?) or various other factors, but these controls are not perfect. Perhaps most women want to breast feed, but some subset have underlying health conditions tied to not producing milk. Maybe that same underlying condition is present in their children, which causes worse outcomes and NOT the lack of breast milk per se. None of these studies can even account for stuff like that because most of them are self-reports and because underlying health conditions may never even be detected.
It's very hard to do good science on humans.
> And really the weight of proof rests on the baby formulas to prove that they're OK.
I agree with that. I do think the proof is there, though. We've had at least of couple of generations of people who have grown up on formula and the studies that compare outcomes never show that formula fed babies grow into significantly worse-off adults.
> Even common sense suggests breast feeding is better, it's been the way people have been growing up for millenia all around the world.
One should never ever appeal to common sense when discussing something about science. Surely you remember that in Aristotle's time, even he considered it to be basically common sense that heavier objects would fall to Earth faster than light objects, right? Human brains are stupid. Common sense means nothing. It might mean less than nothing.
And the sentiment of your statement is also debunked by a single contradictory example. There are many cases in which science/technology has done better for us than nature. Our own immune systems fall flat when presented with many illnesses until we introduce vaccines and medicine. It's entirely conceivable for use to create something that is even better for our bodies than nature has to offer. Nature, after all, is guided only by the selective pressure for us to barely make it to be old enough to procreate. That's all nature has optimized us for, at the end of the day.