A few years back I visited an old water mill called Itaipú. It has forty water wheels still in operation, providing a total of 14 gigawatts peak. At the time I visited, one of the water wheels was providing 95% of Paraguay's electricity, and the other 39 were providing 25% of Brazil's electricity. It turns 50 years old next year. In 02020 its average production was 8.7 gigawatts. They did have to replace one of the water wheels a few years after I visited: https://www.youtube.com/watch?v=thcIrM31tZ4

Brazil uses more electricity now, some 80 gigawatts, so this one water mill only produces about 10% of it now.

It is true that many older water mills are no longer in use, though. Maybe 50 years to you is not old!