"Steel ring teeth make sense; the smaller the ring the more sense they make. I suppose the best arrangement would be a U-channel ring of steel teeth with an inner aluminium ring and arms centrifugally cast into the ring and then machined and peened."

That is the intuitive response, but it may not actually be true. Long ago and far away, back when I used to work for a living as an engineer we avoided putting two hard or two soft materials together in situations where they would slide against each other without lubrication (lubrication being a big no-no in ultra-high vacuum field that I was working in). Much better if one piece was hard and the other soft. This could be as subtle as heat treating one for hardness and annealing the other but identical materials together would bind, gall and generally just tear each other up. You can see this for yourself if you take a stainless steel (not plated) nut and bolt, clean them carefully so there is no foreign material separating the threads then bolt them up tight. They will give you a hell of a time when you want to undo them. If you look at common items - engine crankshafts are super-hard, their bearing inserts are always soft metals. Bushings are bronze or even plastic, while the shafts that turn within them are steel. I'm not making this stuff up.

Things are slightly different in the bicycle drivetrain - a properly maintained cycle will have some lubrication on the chain/sprockets so the the base materials seldom actually contact each other and the correct term for bike chains is actually "roller chain" since the part that contacts the teeth is free to rotate allowing it roll down the tooth face rather than slide. But, in the case that the lubricating film is not adequate and/or the roller has too much friction to roll, the two metal parts will indeed slide against each other and now it is best if one of them is softer than the other. When you hear parts "squeak" that is them chattering unevenly as they attempt to slide one over the other.

I personally wish, from a wear standpoint, that the chain was the only sacrificial item since it is easier and less expensive to replace than worn chainrings, but my personal experience is that an aluminum chainring will have a life far longer than the chain that runs over it providing that the things are kept clean and the chain is discarded before it has stretched excessively.