I agree with your point, but perhaps only when the timeline is short. From Nature:[1]
> Researchers examined the number of known families of marine invertebrates, as well as sea-surface temperatures, over the course of 540 million years of Earth's history. They found that when temperatures were high, so was biodiversity. When temperatures fell, biodiversity also declined.
This happens because global warming causes a marginal increase in tropical areas:
> Tropical ecosystems are known to be Earth's most diverse, and the tropics would be expected to expand during warm eras.
Of course, from a short-term timeline (say 100 years), the rate of species extinction from global warming will surely outpace the rate of species creation, since evolution has no time to act.