Have you actually read The Great Stagnation? It presents a very convincing case. Highly recommended.

Many have pronounced that the era of innovation dead, peace be to its soul. From Tyler Cowen's decree that we've picked all the low hanging fruit of innovation,

Cowen makes it pretty clear that he doesn't think innovation has stopped completely, but that it is far slower than it was during the great era of growth from 1781 to 1970. He also thinks we may have faster growth in the future.

It seems to me that growth is so much slower because the things we demand at these margins are mostly services (healthcare and education being the fastest growing fields). It's much easier to improve the efficiency of a factory than a nurse or a consultant or a university professor.