I don’t agree that there is a technological stagnation. Tech moves in fits and starts, and to the extent that it is true that most of the low-hanging fruit has been plucked, keep in mind that the definition of “low-hanging fruit” changes over time.
But where there is stagnation it’s because we’ve given up nearly completely on basic research, forsaken undertaking large, outlandish projects, and also that the best minds have been driven to finance, law and other well-remunerated but generally-harmful pursuits rather than doing anything useful.
That one factor I think explains a whole lot — if you’re a lawyer busy destroying an innovative company or idea with a patent lawsuit, you’re a huge long-term harm to society. Same with bankers and nearly everything they do.
Stagnation in some areas isn’t occurring because there’s nothing left to discover, to achieve, to imagine but rather that the entire idea of human progress now has got its head pushed underwater and is drowning under the weight of a system that does not value it.