An obvious example of this is the assumption of the geocentric universe. That rapidly leads to ever more mind-boggling complex phenomena like multitudes of epicycles, planets suddenly turning around mid-orbit, and much more. It turns out the actual physics are far more simple, but you have to get passed that flawed assumption.
In more modern times relativity was similar. Once it became clear that the luminiferous aether was wrong, and that the universe was really friggin weird, all sorts of new doors opened for easy access. The rapid decline in progress in modern times would seem most likely to suggest that something we are taking as a fundamental assumption is probably wrong, rather than that the next door is just unimaginably difficult to open. This is probably even more true given the vast numbers of open questions for which we have defacto answers, but yet they seem to defy every single test of their correctness.
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All that said, I don't disagree that technology may be on an s curve, but simply because I think the constraints on 'things' will be far greater than the constraints on knowledge. The most sophisticated naval vessel of modern times would look impressive but otherwise familiar to a seaman of hundreds or perhaps even thousands of years ago. Even things like the engines wouldn't be particularly hard to explain because they would have known full well that a boiling pot of water can push off its top, which is basically 90% of the way to understanding how an engine works.