One the other hand these creature are also a end process of several millions year of adaption to a single niche, some of them may have multible stomachs, it should remembered that even through prmates has the brain size benefit and more evolve social structures, these others creature have adapted to niches for million of years that primates has just begun to adapt to, but in the long term these pseudo-primates are likely to outcompete almost everything.
Perhaps you're right. Aside from aquatic, marine, and some high-speed hunting niches (left for the few surviving Dinosaurs after humans are through with them), this is going to be the First Native Americans all over again. Or for the first time. What odds would you give the early dinosaurs, for that matter? They can run faster than humans, but then so could Saber Tooth Cats.