The estimate of 3/4 largely derives from the fossil record of marine animals. And that means that if a particular fossil species has not yet been discovered, we cannot know if it went extinct.

Our own work does not predict extinction in 2100. Instead it identifies the end of the present century as a time when the total CO2 added to the oceans will likely exceed a value that in the past has been associated with mass extinctions. See also our other comment here.