That's all fine and good, but the modification has to be so profound as to render competing members of the population vulnerable to extinction. It's hard to imagine a scenario like that affecting our species, given the current rate of genetic mixing and lack of isolation necessary for a new and improved variant to develop independently. It seems to me that most change would just be swallowed up by the gene pool at large and all members would benefit or suffer rather equally. Further, many deficiencies and vulnerabilities which might have spelled doom in the early history of our species can now be remedied artificially, so their disadvantages don't weigh so heavily on affected individuals as they once might have.
At the moment, that's completely true. It wasn't always the case, though. The current dominance and huge population numbers of homo Sapiens have come about in the evolutionary blink of an eye, and is not only threatening huge numbers of other species, but many subgroups of our own, while many more have already gone by the wayside. The danger, of course, is that the sudden exponential rise of humanity is but a bubble, and like all bubbles, is subject to bursting. Then the numbers will drop more quickly than they rose, and the vaunted "success" of humanity will be fodder for dark comedy by the few who remain.
As regards competition among species, what advantage does a wildebeest have over a zebra or vice versa? Both species (along with multiple others) utilize and compete for the same forage on the same range, and both (all) appear to be thriving. The principal fly in their ointment is the human presence, which has long demonstrated an ability to interfere, but in the process often disrupt things so thoroughly that it unwittingly destroys its own web of interdependence. The capacity to dominate can be self-defeating.
And there may be a virus lurking out there...
What keeps Wildebeest and Zebras from competing? Maybe they do. But their habitats are not identical -- they just have significant overlap. Their digestive systems are different, and they can concentrate on somewhat different parts of the grazing landscape. And in many ways they are commensualists. The presence of one species benefits the other, in that they are both on the lookout for the same predators. (They have a similar relationship with baboons) If there were no predators, chances are their respective populations would increase to the point where they would be in direct competition, and then we'd see which one had the real advantage. But as long as their numbers stay below the carrying capacity of the range, then any minimal competition between them is overcome by their mutual benefit.
But yeah, I agree that dominance can be self-defeating, and we may very well be headed in that direction. A successful civilization must be vigilant, and not just for foreign enemies. Our success is not guaranteed.