The hardness and decreased ware of the plated bores is just a byproduct of these hard plating. I have not seen anyone talking about the main reason for plating cylinder bores. Iron transfers heat at approximately 1/2 the rate as aluminum.
The engine industry had to find a way to dissipate heat from problematic areas in some high performance engines about 40 years ago. Yamaha began using chrome plating in there road racing engines around 1966. Mahle developed the Nikasil process for Porsche around 1970. Kawasaki developed the electrofusion process in the early 1970s.
As the power levels of stock engines continued to go up, the engine manufactures had to abandon the iron cylinder sleeves. The addition of power valves in two-stroke cylinders created more heat dissipation issues in the vicinity of the exhaust ports making plated bores an absolute necessity.
Replacing an iron sleeve with a plated aluminum sleeve will provide improved heat dissipation over an iron sleeve but will not provide the heat dissipation of a cylinder without a plated aluminum sleeve.
The heat transfer is hindered anytime there is a discontinuity in the metal between the surface of the bore and the water jacket or cooling fins. Even though the junction between a sleeve and the cylinder is a tight press-fit the heat flow through this junction is dramatically hindered.