The issue comes down to basic physics, namely that force is not the same as power (which has the physical dimension of force x speed). In a locomotive, crudely, you need to scale power for top speed, and force for maximum train load (well, as a function of the maximum grade travelled). The force a loco can exert ("tractive effort") is in turn entirely determined by two factors: the weight on the rails and the friction coefficient.

Thus, you can make your four-axle loco as powerful as you want, the maximum weight of a train it can move will remain the same. And, more importantly, you can make its slip control as perfect as you want, your loco can pull less when the rails are lubricated by rain.

Thus, in practice, rather than one four-axle universal locomotive replacing one old six-axle heavy freight loco, two new replaced one old...

_{*Lunatic*, n.
One whose delusions are out of fashion.}