For fine tuning volume and brightness, why not have a single keypress result in a small change while holding down the key results in a bigger change?

Because it doesn't work that way. I was suggesting ways you may get what you need now, because lobbying on HN for Apple to change their hardware and software probably won't accomplish much.

We could all think of things that would make our devices work better, for us, but Apple sells consumer electronics to a global mass market so we have to live with their design decisions or find something else that works better for us.