The question of utilitarianism rests on the opprotunity cost of doing one thing versus another. Sure, some costs are relatively easy to estimate - vegging out on your couch for days watching TV will not likely lead to a lot of good for others (except possibly if you are actively doing research). But since even great investment firms don't always know which entrepreneurs to back, how are you supposed to know whether your current entrepreneurial efforts won't pay off more by reinvesting what you earned into building your business?

Perhaps at the other end, the focus on individual contribution is arbitrary as well. This is also a focus of libertarians who are methodological individualists - who say only individuals act, not orgabizations, and eg paying taxes to have a government agency pay for schooling is always less efficient than a charity that funds private school admission for poor students. But who says? That's quite an extreme position.

Who says YOU can do the most good? Perhaps if they hired someone else to make the money they'd make more and donate more correctly. Maybe you're better off getting out of the way.

Who knows maybe your company when led by someone else can do even more good? Maybe you should step aside after you've built it?

Perhaps giving your time and money to educate a kid or invest in someone else's work will go further because they'll earn more money?

Perhaps robots can do a better job than you.

With more and more technology and automation, it really does seem like there are diminishing returns from a growing class of people, and demand for their labor is falling. Perhaps the best thing people around the world can do is use condoms and prevent overpopulation in their respective areas.

From a purely utilitarian calculus, is it better to have as many kids as you can if you can maximie the QUAYs? What if you could have less kids but they'd all have a higher quality of life? Isn't that better than having a society crammed with poor kids, who as the article points out are nearly as happy as the rich? Is it really additive? Or are we risking an ecosystem crash that can suddenly lead to greater misery?

In short, we don't really have all the answers. I'd advise erring on the side of admitting we don't know what we could so to an ecosystem and not try to rape externalities to satisfy unchecked population growth and growing demand. Ecosystem collapse may be the thing that crashes your QUAYs down, and even if after the whole mess your sum was still higher due to sheer population numbers, I question the additivity.

Finally the problems come down to this: what is the definition of SHOULD?

I believe there is a hidden premise that many people omit when they say should. It is a ternary relation:

A should do B if A wants C to happen

Without individual goals, there is no should.
As for the responsibility of the individual, here is my simple answer and the motto of our company:

People live lives. Companies create products.

Yes it is a bit ordinary and not as catchy sounding as "don't be evil". But it at every point make it clear who has responsibility to do what. When you are working for a project, you create a repeatable product and drive down costs through maximizing re-use. When you are off the clock, you live your life and honor your relationship, which are deeper than just "I don't know you from a hole in the wall." In the marketplace, there are less promises and moral obligations than in an intimate relationship, where shared goals are formed.

Remember - poverty is already going down around the world. The biggest issue we can face today is an ecological disaster caused either by us or an outside factor. Given the history of life on Earth, we can say that the probability of an outside event exceeding what we've seen in the last 10,000 is slim. However, our own population growth in the last 200 years is staggering and unprecedented! We should worry about collapse of various ecosystems that we put pressure on.

At the very least, these utilitarians should read about the https://en.m.wikipedia.org/wiki/Doomsday_argument and stop focusing on mere additivity of QUAYs. Perhaps the best thing they can do is hand out free condoms to everyone on the planet.