Bear with me for a second here, if you will. Let's for a moment assume that alien life forms exist and at some future date they make peaceful contact with us.
As a result of that contact they provide to us a cheap, reliable, safe and efficient source of power, which weens us from the tit of hydrocarbon dependency. The down side is that a disease, carried by and unknown (or not harmful) to the aliens, wipes out 70% of the population, before immunities or a cure is discovered.
Would you then say that the arrival of the aliens was the best thing that ever happened to us?
Well using ETs is not quite the same as the case in the Americas. One thing that's always been inevitable on earth is that those who explore overtake those who don't. In the big picture, even with all the small pox and syphyllis deaths, I still stand by my statement. Even in the most advanced remnents, in Mexico and Central America, they were cutting out the hearts of young girls to appease the gods, as recently as the arrival of the Euros. Where was what remained of civilization headed in the Americas? What would the Americas looked like circa 2005, if the Euros had never arrived, or, even if they had, had left the Americas alone? Think about it.
Old World disease destroyed American Indian society in all respects. By 1648 even the least affected Indians, those living in what is now the Northeastern United States, were virtually exterminated by these diseases.
Oh, yes, the Iroquois won their 300 year war with the Mohicans that year ~ the next they adopted the remnant of a few hundred members into the Oneida tribe. They still exist in the Munsee Band up on Lake Winnebago.
From that year on America belonged to the Europeans, and the Indians who were left went to work as professional meat hunters and guides.
A recemt article in Scientific American concerning the possibility of a new Ice Age points to the atmospheric CO2 drop that may be directly attributed to the sudden absence of over 50 million American Indians raising crops.
It certainly wouldn't be the best thing that ever happened if you were one of the 70 percent that got wiped out. It might very well be a beneficial happening from the perspective of those who survive. A good analogy is the slave trade. A black slave from Africa, taken into captivity and killed horribly on the unthinkably cruel and brutal voyage to the Americas certainly wouldn't have thought the slave trade a good thing. His American ancestors, on the other hand, might perceive things differently. While they still couldn't call the slave trade "good," they could at least acknowledge that its existence benefited them.