Past civilizations did not fail because they slowed down. They slowed down while they were failing. I mean, this is what failing is. They failed at getting enough food and energy to feed their growing population, they failed at managing their growing social and technological complexity. The "people of yesteryear" you reference may have been wrong, until now, for our civilization. But it turns out these people also existed in failed civilizations and guess what, then they were right. Read "The Collapse of Complex Societies" by Joseph Tainter, it's a great book and it makes much more compelling arguments than I ever could about this subject.

Human ingenuity is definitely limited. There are levels of complexity we probably won't manage to get past. More importantly, physical laws have limits, and these are not negotiable. For example, we very probably won't ever get beyond the speed of light. We won't produce energy from nothing. We won't cancel gravity. We won't stop heat death. We won't travel in time. We won't teleport, or beam up as they say. We know that with a relatively high degree of certainty because science has progressed a lot, so we know a lot more about what this universe can do for us, but also about what it cannot do. This is not comparable to a few centuries (or even decades) ago, because then we knew a lot less about both.

We've got 20 years to address climate change and fossil fuel shortage, probably less, before they become catastrophic. There are reasons to think it may already be too late without active measures (carbon capture and so on). 20 years is less than the time it takes to go from brand new technology (let alone lab experiments) to widespread commercial use, which means that technologies that don't yet exist are of no use to address this problem, and that rules out fusion (which hasn't even proven it can produce more power than it consumes, let alone at economically viable scales), among others. I'd say thorium-based fission plants are the only semi-viable bet if we want to continue BAU, because uranium is probably a dead end (there's just not that much that can be exploited with an EROEI > 1).

Note that this is only the energy problem. We also need to deal with over-population, climate change, sea level rise, resource depletion, soil loss, aquifer depletion, species extinction, collapsed fisheries, ocean acidification and so on. At the same time, and at a time when our political institutions are reaching unparalleled levels of passivity and incompetence. If we solve all these problems, remember that our economists and leaders still insist on the need of exponential growth on a finite planet, which means it would soon prove not enough and the new problems we'd face would be even worse.

If you are part of the people who think we're destined to a Star Trek future, I can imagine that the thought of collapse can be painful to you. It used to pain me a lot, but now I'm okay with it. I'd prefer for our civilization to survive, but like with terminal illness, there comes a time when acceptance becomes the only good option.

Our civilization will fail, but eventually the biosphere will recover (though with the amount of damage we do, it'll take more time than with past collapses). Then life will go on for about 500M-1B years, after that it'll be toast and it will most likely be over for life in this corner of the galaxy.