We need to escape the Gernsback Continuum

The 1950s and ’60s are the middle and end of the Golden Age of science fiction

Incorrect. As everyone knows, the Golden Age of science fiction is 12.

Asimov’s stories were often centered around robots, space empires, or both,

OK, this actually calls for a correction on the facts. Asimov didn't combine his robot stories with his "Decline and Fall of the Roman Empire but in space" stories until the 1980s. And even by the '50s, his robot stories were very unsubtly about how thoughtless use of technology leads to social and moral decay. In The Caves of Steel, sparrows are exotic animals you have to go to the zoo to see. The Earth's petroleum supply is completely depleted, and the subway has to be greased with a bioengineered strain of yeast. There are ration books for going to the movies. Not only are robots taking human jobs, but a conspiracy is deliberately stoking fears about robots taking human jobs in order to foment unrest. In The Naked Sun, the colony world of Solaria is a eugenicist society where one of the murder suspects happily admits that they've used robots to reinvent the slave-owning culture of Sparta.

Some dweeb:

I would recommend “Consider Phlebas” by Iain Banks, which is part of the Culture series of novels. Very formative for me, and I read that while I was writing Theme Park. And I still think it’s the best depiction of a post-A.G.I. future, an optimistic post-A.G.I. future, where we’re traveling the stars and humanity reached its full flourishing.

The protagonist of Consider Phlebas is working for the Culture's enemies, a theocratic empire that has slaves literally bred for loyalty, and the conflict they're engaged in ultimately kills billions of sentient beings. Most of the thoughts about the Culture are his, and he basically decries them as the ultimate wokesters. No wonder HN nerds prefer The Player of Games in which a smart nerd like themselves get recruited as an agent to bring down an empire a bit like our own by being really really good at games.

This, and similar writing I've seen, seems to make a fundamental mistake in treating time like only the next few, decades maybe, exist, that any objective that takes longer than that is impossible and not even worth trying, and that any problem that emerges after a longer period of time may be ignored.

It has a valid point that anyone trying to convince you that they can bring a society of the very far future within your own lifetime, if only you do as they say or give them money, is either scamming you or fooled themselves out of desperation to see the end results without the work being put in (understandable when the work required for that society to exist will barring an unlikely miracle of technology take far longer than anyone currently alive now will live), and perhaps that some of this tendency comes from speculating about the possibilities of the future without as much thought to the practicalities of getting there.

It also, I think, is generally correct in that trying to build a more sustainable society is of high importance.

However, at the same time, the kind of future vision it and similar things I've read seem to imply they want people to imagine instead don't really make much sense past the next few decades or maybe centuries. Sustainability is important, sure, but it should be noted that, thermodynamics being what it is, it's also not truly possible. You cannot design a civilization that can persist beyond a certain point without outside input, so if you're trying to think about what paths to take in the future, and value people and societies, it is eventually imperative to acquire resources from outside. This won't bring you utopia forever, but it should bring you more and longer. It also isn't really fair to say that things like space development cannot happen simply because they are very hard, and haven't happened yet despite science fiction sometimes showing it as having happened in the near future. There has simply not been enough time. This kind of science fiction has existed for what? Decades? Maybe a century depending on what you count? Just developing the "easiest" (relatively speaking) parts of the solar system is a task of centuries, getting anything meaningful even barely beyond the solar system one of millenia, and actually controlling the galaxy, assuming it truly does turn out to be empty for some descendant of us to control, one of millions of years, simply on account of the scale of it. It is far too premature to say these things cannot happen just because they haven't in decades. It also is a bit absurd to claim that they won't happen because they simply cannot be done: it is very hard to build a society in a place as desolate as, well, anywhere off earth, and we don't yet have the know-how to do it, but we know that a system capable of sustaining life and civilization over the timescales needed to move through space can exist, because earth is already an example of such a system. Given that it isn't even engineered, it is highly unlikely to be the smallest or simplest possible example of such a system either. We have a society that exists drifting in space already, to replicate and expand it breaks no physical laws, else it couldn't have happened in the first place and I wouldn't be saying this.

There is a certain irony to everyone involved in this argument, if it can be called that. Those who like to think about what could be achieved in the very far future tend, in my experience (I have a very strong interest in such things myself and so hang out in some spaces for such discussion) to have an extremely overoptimistic notion of the timeline, steps and work involved and so seem to think that it will all happen tomorrow, figuratively speaking, without much need to contribute to the actual process of achieving it (or else they have a very grandiose and often counterproductive notion of what that looks like). Meanwhile, those that suspect it's all impossible dreaming that distracts from the immediate problems facing society probably make these things more likely to happen, partly because to build the society of the far future, society has to continue to exist and function from now until then, and partly because knowledge and practical experience in how to maintain a climate, keep industrial activity from destroying it, and not use up nonrenewable resources in a few decades, is also one of the exact kinds of expertise any society built beyond earth someday would need to have. The work of building the future is, for the average person, just in keeping society working long enough for incremental improvements to stack up, and that simply isn't that exciting even if it's the logical prerequisite for much exciting to happen.

It's doubly ironic that many of the people that think that if everyone just listens to them they could do it all quickly, think of themselves as thinking about the long term by doing so. Truely long term planning requires patience and flexibility beyond what any one person is probably capable of at present. At the same time, this doesn't mean that we shouldn't try to imagine a much greater future than what we have, or to put in the work for it, if we can be realistic about what progress we can expect to watch happen and what that work really is.