If everyone in the world were to do their best to maximize population, longevity, and happiness, my vision of an ideal world would have the best chance of being realized. Those who don’t, or actively oppose it, will reduce that chance. Encouraging people to endorse the vision is therefore an important first step in achieving it.
One aspect of the vision that I think all of us would support is increasing individual longevity (especially since it has such a strong correlation with happiness). Historically, average life expectancy has varied between 18 and 67 years. The oldest anyone has ever lived is over 122 years. While we may want it for ourselves, how many of us could be persuaded to want it for others, and what would it take to convince us?
More problematic is getting people to sign on to the idea of maximizing population over time. My guess is that most would be amenable to the idea as it relates to their own families; and less so with others, especially those who are very different from them. I expect that only in cases where the entire present population was in peril would most of us agree to improve the chances of everyone.
The probability of success is perhaps most dependent on the prevalence of altruism in any given population (we would have to convince not just those alive today, but all who will ever be born). From a strictly personality point of view, less than half of us may be expected to be altruistic enough to make a difference.