there are many human ecosystems living below the dam though. Removing the dam means removing entire towns downstream in some cases.

That is not, in fact, true.

I don’t know. I suspect the bias is towards the pre-dam ecosystem so it might be challenging to rationally compare them.

Certainly an ecosystem will grow up around nearly any situation.

Yup my opinion only. To me water is life and if you let it flow to the ocean where it’s only use is to dilute saltwater you’ve wasted a precious natural resource.

Dams does not save any water except the initial filling. Their inflow and outflow match over a seasonal cycle.

Also, they tend to cost water instead: the larger surface area increases the amount "lost" to evaporation.

Perhaps. I suppose that would depend on size and depth of the reservoir and the makeup of the river. Also without a dam the water would be traveling the river mostly during the cold wet season with little evap. So yeah likely so. But in the end the cost would be lost saltwater. Probably a benefit for those that are worried about sea levels.

Yup that’s simplistically true. The difference though they do allow you to choose when it gets released. Around where I live that allows irrigation of crops during the summer months. That water does not go into the ocean. Also around here many of the municipalities draw water downstream from the dam as well. A portion of that travels through the sewer systems, gets purified then dropped back into the river.