It's useful to distinguish, right off the bat, between "moderate" and "radical" longtermism. Moderate longtermism is what MacAskill defends in his book, while radical longtermism is what one finds in all the founding documents of the ideology, including multiple papers by Nick Bostrom and the PhD dissertation of Nick Beckstead. The latter is also what MacAskill claims he's most "sympathetic" with, and thinks is "probably right." Why, then, does MacAskill's book focus on the moderate version? As a previous Salon article of mine explains in detail, the answer is quite simply marketing. Radical longtermism is such an implausible view that trying to persuade the public that it's true would be a losing game. The marketing strategy was thus to present it in more moderate form, which Alexander Zaitchik of the New Republic aptly describes as a "gateway drug" to the more radical position