I see a different approach to this question.
At high speed there is no requirement for a fan because the air flow provides all the cooling that is required. However, the higher the revs, the more power an engine driven fan consumes. So it is essentially wasted. In addition to the wasted power, we pay for it with additional noise.
At idle, there is no flow of air through the front of the car, so some sort of cooling is necessary. An engine driven fan is a simple solution, however, at low revs it does not produce much wind effect. At the same time, it consumes very little power.
Cars mostly overheat at idle, so an electric fan needs to generate the greatest cooling effect at idle. However, this is the time that the alternator is also turning slowly, and has limited capability to produce enough power. The alternator would impose additional load on the car engine, and may well drawn additional reserves from the battery.
At high speed, a thermostatically controlled fan would most likely switch off, and thus consume no power.
Having driven a Jaguar that had twin electric fans, I can attest to the benefit of reduced noise as revs increased, at the expense of more fan noise at idle. When stopping after cruising at highway speeds, the fans would be silent, but would gradually pick up speed as the car idled.
So, your driving habits and climate where you live may help to decide whether you should convert to electric. If you idle in traffic a lot, the electric fans should cool better, be noisier and consume more power. At speed, electric fans would be better than mechanical fans in all these aspects, because they would probably be freewheeling.
Where engine driven fans win over electric fans is where the engine is working hard at low vehicle speeds, like towing, stop start traffic, etc. However, this is also when their noise is most apparent.
No doubt, car manufacturers spend quite a bit of time working out the best compromise to satisfy these different requirements and limit the power usage and noise. To simply go out and buy an electric fan off the shelf would oversimplify all the engineering effort that has gone into the design, and may not fully satisfy the essential requirement as well - cooling.
Hopefully these ideas have given people food for thought.
Previous cars of interest
1976 Jaguar XJS
1974 Rover 3500
1953 Riley (before they became scarce)