That isn't as big a win as you imagine, since while the copper is more expensive, the aluminium is less compact (needs to be thicker to get the same resistance)

That isn't as big a win as you imagine, since while the copper is more expensive, the aluminium is less compact (needs to be thicker to get the same resistance), which in turn means the wires need to be longer for the same number of coils, which means they need to be even thicker to keep the same resistance. That in turn makes the coils much bigger, which also means the steel core needs to be bigger to accommodate the coils, and thicker to keep the magnetic resistance (reluctance) low. Making it thicker again makes it longer means it needs to be even thicker. Basically, one property being a bit worse (electrical resistance of the coil material), makes everything else much worse.