The main fact is that if you gentle treat the varroa, with a "Perlimpinpin" powder or water, you think that you have reduced the number of mites. That's true. But when you think, that your colonies, may cope with a reasonable number of mites, there you are wrong. If the weather, especially drought, reduces the crop, you will see a difference in the brood. Even with brand new queens. And very few mites( let' say less an a hundred). If you keep up with feeding with syrup, things will get worse, because the bees need more proteins to transform the syrup. You will see with a acute eye, all kinds of troubles, spotted brood, chalkbrood, EFB, canniblism, and with an expensive analysis , Nosema Cerenae, IAPV, etc, etc... and finally the famous CCD. Of course you may have colonies dying of insecticids, but that's another topic.
So CCD, in my opinion, and I don't pretend to have the truth either, only experience is talking, is a kind of "Pearl Harbor" of varroa mites, on a deficient of proteins colony. Please excuse my poor english, but you have the main lines.