It's not just cost of a retrofit. Like other structures, the utility of a dam depreciates over time. Unlike other structures, some of that is due to sediment accumulating behind the dam, not the facility itself wearing down. This happens faster than you'd expect - service life for a dam can be <100 years.

Maintaining the facility at full capacity means dealing with the sediment, not just the dam itself. For example, look at what LA County is having to do in the San Gabriels to maintain a damn we want to keep [1].

[1]: https://www.amesconstruction.com/project/san-gabriel-reservo...