I always disliked this 'insight' because what he was describing were features of life that we choose to make terrible: commutes and draining office work due to excessive work days as two examples. Working remotely and being able to take more breaks has dramatically reduced my fatigue, for example, and obviously has eliminated my commute and has been a major improvement to not having a 'dead-eyed' look by 5pm ever work day. He makes the assumption that my selfishness, if applied generally, would be bad for everyone, but I disagree: if I eliminated (or dramatically reduced) everyone's commute tomorrow, I think far more people would be happier than not, even though it stems from my own 'selfish' preference.
I think the biggest problem with the observation is that it equates "the world" with "my world" and does not allow distinction between the two. We each individually, are the center of "my world" and expect "my world" to be organised around my wants, needs, desires and standards. And expectations like that allow us to do things like individually decide to forego commuting in favour of alternative options that vastly increase our individual quality of life, and simultaneously trickle that benefit down into being a more acceptable general option across "the world" in kind, which exactly as you say is hard to describe as any other way than a net benefit to the entire species.
But if we believe and behave that way without understanding the distinction between "my world" and "the world" that is indeed a problem, and we have a word for this old problem that is fairly used as a constant criticism of this increasingly prevalent modern affliction; Solipsism.
While it's hard to argue that solipsism is not a massive problem with the world today and something that is in dire need of being addressed, it's kind of a neat trick if you can convince people that merely optimising their own personal experience of the world by focusing on that experience and acting on their wants with personal autonomy is that exact kind of solipsism, and they should instead constantly defer to everybody else. If one person could do that for the entirety of the rest of humanity, they'd effectively be a permanent ruler by being the only agent in play with actual personal autonomy.
Not that this kind of thing happens, or anything, mind you.
My takeaway from this passage is the simple reminder that we don't have to always assume the worst about people -- and if we consciously choose not to, then even mundane experiences can become generally more pleasant for yourself.
That's part of it, but he's also arguing for this kind of weird passivity towards the way we live which is signposted in the title ('this is water' with the story being framed by two fish). Like, a lot of people get angry while driving because driving in congested traffic really sucks - it'd be better if that mundane experience was dramatically reduced rather than have everyone think that the experience is the water we swim in, rather than a set of deliberate decisions that weren't really ever made with our happiness in mind.
