I do not think that human beings are keystone species. Keystone species are species whose existence are critical to the survival of other species and the environment. Without the species, there would be a drastic change in the environment. So yes the environment would change but in a positive way because without humans, there'd be no pollution, no deforestation, no of killing animals and destruction of wildlife etc. Other species would thrive in their natural habitat with no obstruction. If plants which are the primary producers seized to exist, ultimately there'd be no life. However if humans seized to exist, other lower animals can take up their role in biological processes. For example animals also give off Co2 that plants use during photosynthesis.