I think energy was a poor choice mostly because of its use in a very popular, competing TCG. (Pokemon energy cards). Perhaps a better visible description could have been something like 'fuel', 'gas', 'electricity', 'batteries' or something like that.
"energy" is just the mechanic in-rules name for aether. It is not called "aether" because it most certainly will be used in future sets outside of Kaladesh. So it's just mana and aether. And aether is a concept that existed many many years ago when Magic's "Multiverse" was conceived. Aether is the thing that exists between planes, so you could say Planeswalkers travel through aether when they "planeswalk". Also, aether is the "place" where Eldrazi actually exist.
