I think the issue is more not killing everything in the river the warmer water is released back into, so the temperature differential becomes unworkable at certain intake temperatures.
I agree with the intention. This has been part of the design constraints of a plant for at least 30 years... I reckon it is possible older plants didn't take steps to cool down the open water loop, but it is indeed relatively straight forward to tackle: cooling towers, open air reservoirs, and again heat exchangers...
It's not an easy thing to retrofit into an existing plant though: first of all France isn't the US and even though it's the biggest country in western Europe, it's relatively space constrained, especially in the places where plants are built, so most exiting sites are already quite cramped (even just post-Fukushima external diesel generators gave headache to my wife's colleagues in her plant site). Also in France we have a lot of reactors, on very few rivers (Rhône and Loire river have like a dozen reactor on each) so there's less margin than what we'd like.