I don’t see why extinction could not be somewhat gradual. For example, a number of us here think that Firestone et al. are right about a Northern Hemisphere boloid strike(s) around 13,000 years ago. This also coincides with the death of many of our mega fauna. However, it is also known that Mammoths and some others were still around several thousand years later, if not here, then in Europe and Siberia. I think that many dinosaurs were destroyed by the immediate impact devastation. I think the direct burns of the fiery radiation killed many. This even probably also destroyed the ozone layer so that skin cancer killed many survivors. I believe this is why small jungle/forest living mammals survived, as did snakes, turtles, crocodiles, and other creatures that were not running around in sunny open spaces. Nevertheless there were probably some dinosaurs protected by being asleep in a forest/jungle, on the safe side of a hill or cliff, or in a canyon that did survive for a time, but as the climate continued to stay bad for a long time gradually all died out.
Extinction is instantaneous — a species is alive, alive, alive, alive, then one half of the last mating pair dies off, still alive, alive, then boom, last one dies, and the species is instantly extinct.
For extinction to be gradual presupposes that the biome has psychic powers and knows where this is heading. It’s an outgrowth of Lamarckian nonsense like “selection pressure”.