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There's no evidence that tech is stagnating but quite a lot that the pace is accelerating, perhaps geometrically. There are two reasons-- one is the integration of many more people into the western/global structure of scientific and material progress (particularly in Asia and South Asia) and the second is the complementary impact of advances across multiple fields. Consider for example, the enormous impact of internet communications tech on medical research.

The Wright brothers, working in a bicycle shop, invented fixed wing flight in 1903 and it took less than 100 years for western civilization to develop jumbo jets and space planes.