A flying foe that closes to melee range is already risking OAs each time it breaks contact, unless it has flyby, which is rare. If you are the person being attacked, you can drop a Stunning Strike on that OA if it hits. It costs half the ki and is utterly devastating if it works. If you're not the person being attacked, there is no guarantee you're going to be within 30 feet of that person, either.
As I've said in this thread, flying enemies usually have a reach larger than 5ft, disabling OA from stunning strike. A monk should be near their ally spellcasters, since they're the prime target to be attacked with their weaker defenses and the fact they're high damage. Their priority. But if they're 10ft away from the spellcaster you're adjacent to, they're at least 30ft away from you.
How often do you face flying humanoid spellcasters?
As a player? I've never been in a campaign that hasn't had a flying humanoid. As a DM, next session my players are going to be fighting cultists riding giant bats as mounts. My party is level 3 so they can handle it.
Okay, hang on a second. That's three different disciplines you've tossed out here, one of which isn't available until 11th level.
Yeah, that's how many you get at level 11. Actually you get 4 disciplines at 11. Before level 11, you could use your elemental attunement to create makeshift darts. Or use a shortbow. You're basically doing that for open hand and shadow monks, too. At level 11, though, you get a massive mobility buff.
This is what, in wizard theorycrafting circles, is known as Schrodinger's Wizard: The wizard whose prepared spell list is indeterminate until there's an encounter, at which point the waveform collapses into whichever spell just happens to be perfectly suited to that encounter. And wizards get many times the number of prepared spells that monks get disciplines. Are you going to devote every single one of your discipline picks to dealing with flying enemies? If so, that's not exactly a compelling case for the Four Elements monk.
I mean, yeah, I would. I wouldn't say 4-Elemonks excel against flying enemies if I didn't spec like they did. Here's exactly how
I would build: I'd take Unbroken Air at 3rd because flying enemies with high strength at this level is rare. 6th level, I'd get hold person, for those spellcasters. At 11th level, I'd switch EA for fly, I'd take fire snakes to extend my mobility and reach to basically anything in any battle. At 17th level, I'd take wall of stone and trade my Clench of North Wind with cone of cold.