One thing that affects a person's motivational structure is the rational or irrational belief that "the world is ending". Another way to look at this is that there is a certain field in which one can work, and if the field will be there, it's worth working in it, and you are motivated. But if the existence of the field is uncertain, you start to feel doubtful about working in it, and you lose motivation.
"World-ending" can come on a spectrum. The strongest world-ending would be the non-existence of all that exists. But people don't worry about that, and I don't know of a reason to think that will happen. I can think of a couple of less-extreme but still decisive world-endings, that have some basis to be foreseen. God (or the metaphysical organism) might see fit to end the current system, the current world. We would survive, but assuming that work we do now would make a difference in 100 years the way we thought it would now wouldn't make sense if this happened. A second is, we invent artificial intelligence that can change things so fast, so out of control, according to thought processes so foreign to us, that while we might exist after this rapid change (this "Singularity"), efforts that we expend now might have little to do with the way things are after it.
Neither of these two world-endings is certain to happen in my lifetime. But I can sense them affecting my thought process, from time to time. They can be demotivating. One of the best ways to motivate yourself to work in the present is to make a plan for the future. Suddenly, you will have a lot of intermediate steps to work on, in the present. I think our culture (or, a lot of it) has forgotten the future or never saw it, so that now we don't reach for the future and make the present better in the process. When I am most liberated from the present-only mindset, I can see numerous opportunities to make the present or near-future better. I can subjectively assign the probability of the world ending each year to be some low but non-zero value each year and conclude that it is rational to work for the present or near-future. But when I am stuck in the present-only mindset, these things which I might do, even for the present, disappear to me.
I think I remember reading an anecdote about someone who worried about nuclear war in the 1950s and thought that worrying about bridge design didn't make a lot of sense, but then decades later was grateful to the people who did think about bridge design, since nuclear war hadn't happened to that point, and a person needed to drive on bridges after all. (I want to say the anecdote might have been about Richard Feynman.) So we might hope that someone cares about the future even though it might not come. What if the Singularity, or the Apocalypse, or the Changing of the World, don't come for another 100, or 200 years? With respect to my lifetime, I then need to think long term, and there is a place for long term thinking, to address the field of earth in the 2100s or 2200s.
I think the MSLN answer to this question is to say: "What is most important is not altruism, but to be an altruist. An altruist necessarily cares about effectiveness. But being an altruist, to the best of your ability, or in whatever other way necessary becoming like God in who you are, is necessary at all times, no matter how futile things seem." Your horizon is always far in the future, with respect to your life on earth, and so if the things that you do here end up being mere gestures after the Singularity gets through with them, or after the world is ended by God, then at least you, with your inner being, survive to the next world. The field of you, of who you are, remains relevant, regardless of what happens with the outside world. So, if you have the time, work in whatever way you can, and if that way that you work presupposes that there is a relevant future, live that way, so that the imperishable part of you can certainly be invested in.