> There is no absolute ranking. Individual meaningfulness is all that exists.
This is not even consistent. Even if you admit only individual meaning, then certainly there are aggregates of those values across society. And advertising is not as high as feeding the hungry or contributing to world peace, as obvious examples.
Collecting stamps may be meaningful to you as an individual, but society also judges your contribution as more or less meaningful. Psychologically, your own experience of meaningfulness is influenced by the opinions of others.
So you can see already that even if you disregard any questions of morality in how you spend your life, the idea that meaning is purely individual already breaks down and becomes inconsistent. We are social animals.
That is a wonderful letter and an excellent example of how the cost of space exploration can be justified. Advertising, while wasting more human resources, does not need this kind of justification, because it is not usually funded by public institutions. However, if you were to try to write such a compelling letter to justify the vast human expenditure of energy and intelligence on advertising you would find it difficult.
The letter makes specific examples of the benefits to all of us on Earth of how space flight requires technological progress, which improves our well-being, and how space exploration inspires us to better appreciate and protect our planet. You cannot replace space travel with advertising and make the same arguments.
Whether space travel or curing cancer is more meaningful, it's certainly good that we are doing both. However, the same is not true for advertising. If you took 99% of the talented and capable people in advertising and put them to work on curing cancer, feeding the poor, providing health care or education, or any of a number of other areas, society would be better off.
Some people derive meaning from making a good living and supporting their families. If advertising is the way to do that, great. But if you derive meaning from the value that your work provides to society (and to some extent, everyone who works does) then advertising (in the majority of cases) is not for you.
If we could disarm all the world's armies overnight and prevent all wars, and spend those resources on something else, wouldn't be better off? We cannot, because the competitive structure of our world is such that unilateral or universal disarmament are both impossible to achieve. Military spending and war remain necessary because we have found no way to move beyond them.
If we could create a world overnight with no advertising, we would be better off by all the productive areas that energy and creativity could be redirected to, but we can't because of the competitive structure of our economies. Advertising is made necessary by our current system, but that does not mean it is equally socially valuable or provides the same meaning to those who work in it as every other field.
In making this comparison, I mean no disrespect to the military. You can agree that war is horrible but still serve from love of country. There is nobility in that. I don't think you can make the same claim for advertising.