Your take on this... is valid IMO... IF evolution is even partially true.
I do believe in micro-evolutionary change... or adaptation if you will... which would also lend itself to your post.
I do not believe in macro evolution... the morph of one species into another over time.
It is a trade off... vegetarians have a lower risk of some cancers, but a higher risk of others such as colorectal. They also have a confirmed higher risk of Alzheimer's... this has more to do with a lack of healthy dietary fats I believe than anything else... and perhaps heavy metal toxicity, such as mercury which is implicated here.
The increased risk of Alzheimer's would make sense with vegetarians due to the lack of animal fats, B12 and protein which have all been deficiencies noted in people with Alzheimer's. These things can be dealt with sufficiently enough in a vegetarian that understands these things and makes an effort to get these nutrients in other ways. Not impossible, but again enabled by modern agribusiness.
Point I am trying to make here as well, is that we must all find our *sweet* spot in terms of dietary balance... we are all different, live in different environments, and are dealing with different environmental toxins, and thus have different dietary needs in order to maintain health and well being.
As an example... If I lived in Minnesota, a banana, imported from South America would not likely be a *great* food for me - perhaps acceptable... but local, in season fruits... would be another story.