Turning into an air elemental is still a form of closing with the enemy. And it leads to bad things when the dragon knocks you out of wildshape.
You're right that druids do have some short-ranged spells like Call Lightning (120' IIRC), Produce Flame (30' range), Thorn Whip (30' IIRC), etc. But past that point they're reduced to plinking away with one attack per round on a longbow or crossbow--and they're not even proficient in crossbows/longbows!
In a campaign full of dragons in flight, Moon Druids will be relatively unhappy compared to e.g. Sharpshooter fighters. That doesn't make Moon Druids in any way a bad or weak class--in fact, the narrowness of the conditions ("a campaign full of dragons in flight") highlights the strength of the class. A DM has to go pretty far out of his way to make Moon Druids anything but a powerful pick.
Level 14 Moon Druids useless? Hahahahaha. Regenerate (one nigh-unkillable PC for an hour), Reverse Gravity (large AoE crowd control + damage against anything without flight, targeting a weak save), Conjure Fey/Elemental + Planar Binding (e.g. Invisible Stalker for 1000 gp each, or a nice coven of invisible Green Hags for Lightning/Polymorph/Counterspell/etc. for 3000 gp), or just summon a nice flock of 24 Giant Owls/etc. (if you can't think of a way to use 24 Giant Owls, you're not trying hard enough. E.g. 98.5% chance of successfully grappling an adult red dragon and causing 20d6 falling damage to it).
And it's not like you don't still have good old lower-level fallbacks such as Polymorph, Pass Without Trace, and even Faerie Fire (targets a weak save + gives advantage to allies = solid).
Unlike a wizard, I don't even have to guess whether your level 14 Moon Druid had access to these spells. Because he's a druid, I know he had these options. Semi-useless? No way. If your level 14 Moon Druid was semi-useless, either your campaign is atypical (lots of highly-mobile spellcasters and solo monsters maybe?) or your Moon Druid player was playing badly.
RE: elemental form, Mobile feat + (Earth Elemental Earth Glide or Air Elemental flight) never goes out of style. CR 5 forms like Hulking Crab and Giant Crocodile/Brontosaurus are also good.