This is a massive oversimplification, just like saying the opposite is.
It’s not, at least certainly not in all cases. The material for brass/copper does have a better thermal conductivity coefficient. It depends entirely on the particular construction of the radiator in question. For some applications aluminum will perform better, for others brass/copper will. It will literally some down to the construction of the individual radiators you’re comparing.
You’re making a ton of assumptions here. Just because the OEM’s have all gone aluminum/plastic doesn’t mean that’s the best for a particular application.
Aluminum is cheaper, which is a factor. Aluminum/plastic is a lot cheaper. It’s also lighter, so for overall mass reduction you can be looking at better fuel efficiency which is a benchmark that the OEM’s have to hit. Aluminum radiators can typically also withstand higher pressures because of their construction, which is also very helpful because it raises the boiling temperature on newer cars that work better when run hotter because of other requirements.
As for the rest of it, the best cooling system is one that has components that are matched to work together. Just slapping on a higher output pump isn’t always the answer. Running the pump at the speed it was designed is best, so underdriving or overdriving it can both result in issues. And whether or not a particular engine’s water pump is under or overdriven doesn’t tell you much, sure, you can look at the pulley size and see if it is. But just on our cars Ma Mopar ran both under and overdriven pulleys on literally the same model of engine depending on whether or not the car had AC. And further, used two different pumps, one that was under driven and one that was overdriven. So unless you already know that was stock seeing a pump that’s over or under driven doesn’t tell you anything.
If you’re running a mechanical fan, having some kind of clutch fan is a GOOD thing. Working properly it should only reduce the speed of the fan when the car is moving, and they airflow from the speed of the car is doing the work so the fan doesn’t need to be. A direct drive mechanical fan is just wasting power when the car is doing anything much better than 25 mph. Which is one reason that electric fans are a great improvement in efficiency, because you only run them when you need to. If you need the fan running 100% when doing highway speeds, you have another issue.