I think there are two issues here which are only partially intertwined.
- Is the US in decline as a world superpower/empire?
- Is the entire planet doomed to decline due to the inevitable exhaustion of cheap energy from fossil fuels and/or climate change?
In regards to the first point, I would say “yes”. Viewing the US from a historical context it has, in fact, reached a peak of influence and is declining. We see this internally from a cultural standpoint and externally from the increased cost of maintaining military and economic superiority (ie empire maintaining). The US is rather efficient is the way it manages its “constituent states”. It prefers means like currency manipulation, IMF loans for resource access, super secret coups, and only uses more expensive outright military options when all others fail. Still, it’s becoming too expensive to maintain when the citizens continue to selfishly demand higher standards of individual living. The eventual “nails” in the coffin for US super power status is yet to be determined. I can think of a few things that would hurt; loss of world currency status, collapse of free trade agreements, ever increasing internal strife. I think its likely to be a combination of these and other, not yet predictable factors. The end result may only be a weakened, but intact US, or it may divides along ideological or regional economic lines, but again, not predictable. So the question becomes will this happen in the next 30-50 years? I think it’s possible. More possible than worldwide nuclear devastation or zombie apocalypse. Decline, however, is much more likely than reemergence to peak power and influence.
I am not qualified to address the second point effectively. I simply know that a great many people, much more intelligent than I, have deep concerns about peak oil. My amateur opinion is with the current trend towards renewable and a large supply of natural gas and coal remaining for relatively efficient electric energy (with grids already in place), we will likely see a transition in which fossil fuels the developed world uses. Increases in energy cost, likely increased volatility in prices, but not world devastating in the near or midterm. At least, until these resources begin to dwindle. Who knows, by that point we may have zero point energy or some such scifi solution. Climate change being another world wide issue I’m unqualified to speak towards, but it will also likely add costs and volatility to world food production. Together, my guess is the next 100 years will see much less growth and productivity gains than the last 100, any gains will likely be used to simple to maintain status quo. I doubt, however, this will be enough to hamper my personal ability to “live the good life” for the next 40 or 50 years as long as I remain attuned to these changes.
Simply because I believe the US is declining and the world will have some serious challenges ahead does not mean I’m a “doomer” or “prepper”. In fact, I’m optimistic these transitions will result in a better world for humans down the road. I’m also optimistic about my and this community’s ability to deal with these issues as they arise. I simply think the transition periods will make the next 100 years a bit tougher than the last 100. Having an enjoyable hobby that may help my FIRE go more smoothly in certain possibilities isn’t crazy or pessimistic, it's fun. I wouldn’t judge someone who chooses different hobbies. However, I would argue being optimistic to the point that one does not see changes in the US and the world, if they begin to happen, will have a huge disadvantage.
Look to the world of biology; over the long term, species that are the most adaptable are better off than those who over specialize to only one particular environment. Although, the latter may have an advantage in its specific environment. So I guess the question is, will things change or remain the same?