The always-entertaining Onion A.V. Club has a rundown on great (or not so great) movies' twist endings, starting with the unforgettable Psycho and ending with last year's already-forgotten Hide and Seek. They include the mandatory two Charlton Heston movies, including Soylent Green, but offer this put-down of that film's Edward G. Robinson-mashing twist.
Could it work today? No. Awakened to the precariousness of the environment by early-'70s activism and films like Soylent Green, the world put the brakes on overpopulation and the exhaustion of natural resources. The air has never been cleaner, the climate's never been healthier, and corporations never put profits over people.
Cute, but Soylent Green is a camp classic because this stuff is true. Early-'70s activism, which I'm going to go ahead and re-interpret as "Paul Ehrlich," completely hyped and overestimated the rate of world population growth, the effects of pollution, the amount of oil left in the ground, etc. Early '70s sci-fi like SG that was informed by this thinking is a laugh riot, even more amusing than the 1980s nuclear hysteria that gave us the greatest disaster film of all time, Threads. But that's another post.