It's not about being remembered, it's about your life having positive impact beyond those you can personally see.
Why do people want to make a lasting change? Purely for intrinsic reasons? Or is there some hidden assumption like for example that it will make them happy in their later years?
Yet you seemed to want to draw a distinction between happiness and a lasting impact. Though the distinction was perhaps just with the family/local impact.
> I also don't think there's some kind of "altruism" that isn't ultimately just another way of getting happiness - maybe a more intellectual one, out of the feeling of doing something one considers morally good.
Right, but a family-related happiness is less because it's just a result of chemicals (oxycotine?) that are related to survival, a sort of failure mode? You said yourself that it is less fundamental. What is fundamental? And why is your derivation of happiness higher than those who go by the supposed more base instincts of focusing on their family? Because the hormonal causes are harder to pin down and analyse? So what? Something more complex and harder to describe is better?
If you feel happy, I think the distinction of why is unimportant.
If you were born into a very tough situation in a war torn or third world country, you'd do what you needed to with what was given to you. While we do have a lot of material wealth and might have more free time, we still face the same great equalizer: death. As do our civilizations and our planet (and likely us, way before that). Given the constraints, you can simply "try your best". Living without regrets might be a job to one person, a 90 hour startup to another, traveling and learning to others. Whatever your belief is, following it is the best that you could do. Ultimately you have to answer to yourself & be able to live with yourself. Personally, living selfishly, while it maximizes certain things simply goes against my core beliefs. I don't necessarily know that living less selfishly is necessarily better. But for me, it is. Your beliefs, may defy logic, and may not necessarily maximize legacy, wealth or whatever, but in having acted you will have done your best given the situation to be able to live with yourself, which is all you can ask of yourself.
If you have to be less rational to accept an answer, doesn't that imply that the answer is wrong?
Oh, wait, everyone with a scientific turn-of-mind has already been labelled a "robot". Beep boop, then /s.
The world I see does indeed need to improve a bit, on all possible levels. If you strive to improve mankind as a whole, or at least big parts of it, please go ahead and do your best. Just don't neglect that personal, "local" aspect as so many do these days (ie crappy parents raising even crappier kids). This world also needs happy, balanced and positive people, not only depressed, exhausted overachievers (or overaimers) I see so often around me.
I tend to focus locally (friends, family, coworkers), and let the effects seep wider if possible, on their own. One example - 2 weekends ago, I took my girlfriend to Mont Blanc with guide, on skis, and on top, after gruelling march on skis, I surprised her proposed marriage. The reactions from people around, but also total internet strangers were overwhelming, quite a few wrote that in these (for them) depressing days, seeing this gave them positive energy and hope (for life, relationships, people around etc.).
Did I change the world with it? Of course not, I am not the first nor the last guy that did it. Does this have some, maybe minor but positive impact on other people? Definitely yes. Enough drops in the pond can also have an effect.
Impact exists with our without legacy. The network effect of your friends' lives may be outwardly invisible, but the impact of their work, on the lives of anyone they lived near and around, had fun near and around, served near and around, worked near our around... is quite vast.
You don't have to be Abe Lincoln or John Rockefeller to have lived a meaningful life.