@Kenny
Oddly, I always remember reading that the transformer was one of the more efficient devices in terms of energy loss (IIRC, a good transformer is supposedly >95% efficient). But as you point out, they do get warm and they do hum occasionally.
What sort of efficiency do solid state transformers (as shipped with mobile phones etc.) have, anyway?
Incidentally, the transistor never actually replaced the diode - they're used for different purposes. Unless you meant vacuum tubes, of course (of which there were diodes, triodes etc).