It will be a miracle solution until the elevated water temperatures start affecting the ecosystem downstream and potentially the health of the river as a whole, if not the effectiveness of the system itself. Then people will realize that actually perhaps it wasn't so miraculous after all, and like all engineering choices, is a compromise between what we're getting from it and what we're giving up. We could even point at the new algae problem in the reflecting pool as an example of such unintended consequences from heating the water above traditional norms. For some of the same reasons you shouldn't dump raw sewage into the river, it's not an infinite garbage disposal, it can't just endlessly absorb your waste whether it's domestic, biological or heat or anything else. You're not getting rid of it, you're just making it someone else's, somewhere else's problem, potentially at some other time. But maybe it's a reasonable solution for right now, it might be better than the alternatives, maybe the tradeoffs are pretty minor or even legitimately ignorable when weighed against the benefits. But as long as we cling to the insane idea of endless, infinite economic growth, they're only ignorable until it that growth makes it inevitable that someday they're not, and calling it miraculous feels a lot like pretending there aren't any downsides, and I don't really like that in principle.