Water which is evaporated is also returned to the ecosystem. It falls back out as rain.

The heat issue is also widely misunderstood. The state of California gets on the order of a PWh of heat every day from the sun. A million GWh. Even a large nuclear plant doesn't make a dent in that. The issue is localized to the immediate area at the outlet, because the water temperature directly at the outlet is going to be higher than it is at the inlet, before it diffuses back into the background. Naturally the way to mitigate this is to pull more water through the cooling system, all of which you're still returning, but then you raise the temperature of twice as much water by half as much temperature, which is what you want since the temperature delta is the concern. Of course, doing this will cause people to accuse you of using more water, even though you're neither consuming the water nor modifying the amount of heat being transferred to the environment.

In addition to that, water has extremely different levels of scarcity depending on what region you're in. Parts of California and Texas have arid climates and extremely limited economical means to source additional fresh water. Areas like the Midwest get dramatically more rainfall and as a result fresh water is not meaningfully scarce there.