You seem to believe I am a staunch proponent of hydroelectric, but I am just asking you to give examples that most dams have a life span of "decades or less".
Your own link, says "Nowadays, from what structural engineers tell me, the typical design life is more like 100 years".
Swedish dams, some which are 100 years old already, are getting upgraded. This example, on a 50 year old dam, is supposedly prolonging the lifetime by 100 years.
"Although large concrete gravity dams have a theoretical design life of 80-100 years, the actual lifespan of a dam is determined by the rate at which its reservoir fills with sediment. In severely eroding catchments, millions of cubic metres of sediment can be transported annually. The average lifespan of a large dam in China is 45 years. "
http://mightyclutha.blogspot.com/2010/02/decommissioning-rox...
"Many scientists predict that within two decades, rising demand for Colorado River water and falling supply will drop the surface of the reservoir to its lowest level, known as “dead pool.” Because the lowest exit from the dam (the river outlet works) is 237 feet above the original riverbed, at dead pool Lake Powell will still hold 2 million acre-feet of water, one-thirteenth of capacity. "
https://orionmagazine.org/article/calamity-on-the-colorado/
And on, and on, and on. The takeaway is, the silt and sediment that collects on one side of the dam drastically shortens the usable lifespan of the dam, no matter how long the structure itself will be structurally be sound. That's a total engineering fall.
Your own link, says "Nowadays, from what structural engineers tell me, the typical design life is more like 100 years".