First, let me say that I usually start by mentioning the mass extinction, because technical people (myself included) have a tendency to believe that "the problem is CO2 emissions, and we can find new technology to reduce that". So I find it important to note that we humans are already responsible for the loss of essentially 2/3 of trees, 2/3 of animals and 2/3 of insects on Earth, and this is before the consequences of climate change. It is easy to say "if we had more energy, we would grow crop vertically and leave biodiversity alone", but sadly I don't think it works like this. Crops are not the only factor (population raises because of cheap energy, so we transform more wild spaces, also for tourism, and I am not even talking about hunting and industrial fishing, etc). But even for crops, we already know a simple way to use them more efficiently: eat crops, not meat. But we don't do that, because with cheap energy (and ignoring the fact that we are killing the planet) we don't need to. If you think about it, we used to eat the crops (and not the meat) in the past, when we did not have access to cheap energy :-). Why would we suddenly avoid rebound effects with fusion, when history shows that it has never happened for any technology before?

Then, if we put the biodiversity problem aside, there is the energy problem. Modern society fundamentally relies on fossil energy, which is limited by definition (this is very important to realize!). We passed the production peak for conventional oil in ~2008 (Europe has been feeling it since then), the global peak is predicted to be around now (I remember the predictions for the peak of conventional oil from when I was studying in 2004, and retrospectively they were very accurate, so I could imagine that the new ones may be reasonable as well), and for natural gas it may be in the next decade. So fossil energy will become a problem in the next few decades. Building a nuclear plant today, with technology we know (i.e. fission) takes what... 15 years? Fusion is most likely already too late for the party (and anyway it is not clear to me that it fundamentally solves the problem better than fission, except that people are afraid of fission for some reason). But nuclear plants make electricity, and that's only part of our society. Planes don't fly with electricity, plastic is not made of electricity, and we can't store electricity in big jars like we do with oil. So we are facing an energy problem, and we don't know technology today that could remotely solve it (I know, it's hard to accept for engineers, but please try to seriously think about it and don't stop at "many VCs put a ton of money on startups that try to find solutions, it will work out in the end").

Finally, and on top of all that, climate change is coming (again in Europe we have been feeling it strongly every summer in the last few years, and it won't go back to "normal" ever again). From what we can observe, it seems like the IPCC predictions are systematically optimistic; in other words, it seems like climate change is going faster than predicted. So it's not that we can count on 60-80 years to solve the energy problem anymore: now it seems like we may be playing the "survivability" (if that is a word in English? :-)) on many places on Earth in the next 10-20 years (maybe less). As a reminder, 10-20 years is the time we need to build a nuclear plant. If we continue in the next two decades the way we have in the last two decades (which we are clearly doing, if not worse), it is very likely that we will have screwed up many places on Earth for the next centuries. That means mass immigration, famines, wars, general world instability... not very good conditions to develop new non-military tech to solve a problem we haven't solved in decades of peace (globally).

Those are all problems with huge inertia (i.e. once you realize it is very bad, then there is no coming back), each with major consequences if they were taken individually. But they are not happening individually, they are happening together! Essentially for the same reason: our society is based on abundant energy. So the solution is not to "make a small step in one of those problems with a major technological change (like fusion)", because (and ignoring rebound effects with new technologies) even if we solve one of them, the other two problems will screw us big time.

All that not to say that we are already screwed anyway: maybe it's too late already (that's possible), but maybe not. And in case it is not, the highest chances of survival IMHO come from degrowth. Degrowth is a huge technological challenge. It's maybe not as fun as throwing Python scripts using ChatGPT in piles of docker containers running on hundreds of machines on the cloud, but it is not less challenging: how do we do less with less, while keeping our society from collapsing?