IMHO Steam Machine should stay
because it fits better these elements theme, but if that's not enough I can only add that it wouldn't work as good in air.
Ok, air and fire do have a synergy because of unstable gas, but unstable gas decks are aiming to be fast, so they would work better with other air creatures for support. Air is not a very defensive element and it's only way of stalling is to kill creatures with eagle eye, shockwave, unstable gas/thunderstorm. The fog shield might help a bit with stalling but not enough. You might try to use fire for more creature control but you would still lose to anything with tough creatures (light, gravity, some earth decks etc.)
I guess in water Steam Machines would be much more useful because permafrost + congeal + squids + flood + trident (if you add earth) offer a better stall deck giving you time to pump Steam Machines.