This is the dumbest comparison, generalisation and statement i've heard in a while!What happens when the wire size is undersized and cannot pass the amount of current that the amplifier can safely take? That's why you fuse for the wire, not the amplifier.
By totaly different scenario you chose what to do in another. Maybe someone is going to read this and think he is safe to install a system with underated wire if he fuse it by the wire rating.
This is just plain wrong thinking and installation! @ItsonlyaHONDA is good example trying to run 400A throught 0AWG wire.
Overrated wire gives you better voltage drop index which most people in car audio are hunting for.
Here we are speaking about 80A fused amp and non classified 4Awg wire. For starters lets believe that this wire is able to push 140A. Did any of you actualy take in to the account how long is the wire or does it go through the engine(heat). If you actualy insert this into the ISO or ABYC calculator you would see how current drops. One more thing wire current rating is max rating and fuse needs to be at least 20% ->this actual electrical rule.