> The teenager is making an implicit moral judgement, with themselves as the only moral patient.
No they're not! You have made a claim of the form "these things are the same thing"—but it only seems that way if you can't think of a single plausible alternative. Here's one:
* Humans are motivated by two competing drives. The first drive we can call "fear", which aims to avoid suffering, either personally or in people you care about or identify with. This derives from our natural empathic instinct, but is can be extended by a socially-construction of group identity. So, the shrimp argument is saying "your avoiding-suffering instinct can and should be applied to crustaceans too", which is contrary to how most people feel. Fear also includes "fear of ostracization", this being equivalent to death in a prehistoric context.
* The second drive is "thriving" or "growing" or "becoming yourself", and leads you to glimpse the person you could be, things you could do, identities you could hold, etc, and to strive to transform yourself into those things. The teenager ultimately wants the PS5 because they've identified with it in some way—they see it as a way to express themself. Their "utilitarian" actions in this context are instrumental, not moral—towards the attainment of what-they-want. I think, in this simple model, I'd also broader this drive to include "eating meat"—you don't do this for the animal or to abate suffering, you do it because you want to: your body's hungry, you desire the pleasure of satiation, and you act to realize that desire.
* The two drives are not the same, and in the case of eating meat are directly opposed. (You could perhaps devise a way to see either as, ultimately, an expression of the other.) Human nature, then, basically undertakes the "thriving" drive except when there's a threat of suffering, in which case we switch gears to "fear" until it's handled.
* Much utilitarian discourse seems to exist in a universe where the apparently-selfish "thriving" drive doesn't exist, or has been moralized out of existence—because it doesn't look good on paper. But, however it sounds, it in fact exists, and you will find that almost all living humans will defend their right to express themselves, sometimes to the death. This is at some level the essence of life, and the rejection of it leads many people to view EA-type utilitarianism as antithetical to life itself.
* One reason for this is that "fear-mode thinking" is cognitively expensive, and while people will maintain it for a while, they will eventually balk against it, no matter how reasonable it seems (probably this explains the last decade of American politics).