Why not only flight? I mean, there are a bunch of different kinds of flight, it's more likely to be encountered and is generally a bigger part of the game overall. You are more likely to encounter flying enemies than swimming ones, and typically speaking you are more likely to engage them while they are flying compared to enemies who swim. Plus with swimming the bigger issue is not the mechanics of movement but rather being in a substance that can potentially suffocate you.
Okay, so flying over swimming because swimming is less common. Why not ground-based movement? There are a bunch of different types of ground-based movement, and it is even more common and even bigger in the game than flight. So it should absolutely be changed to reflect that then, since we must change the rules of flight.
But I find your last line interesting. You say the the "bigger issue is not the mechanics of movement but rather being in a substance.." Now, to me, that reads like for flight the biggest issue IS the mechanics of movement... but why is it an issue? I've been running these flight rules for nearly ten years without a single issue, so how is it an issue?
Literally the entire point of a strafing run is the mechanics of flight: you strafe because you can not stop and your weapons are pointed forward, hence why you continue through and strike a beaten path rather than continuing to focus on a single target. Contrast that with, say, a pylon run where you turn over an enemy because you have access to side-mounted or turnable weaponry, or how helicopters can maneuver and fire.
So a knight with a lance is also a strafing run, because of the mechanics of a galloping horse. You can't just stop, and your weapon is pointed forward. In fact, have you ever once seen a galloping horse charging in a straight line, then just immediately stop without slowing down? Now, have you ever seen that happen in a DnD game? In fact, you could have your horse dash and gallop, hit someone, then immediately take a 90 degree turn to avoid trampling an innocent child and continue moving at full speed. At least, in DnD, I wouldn't recommend trying it IRL.
But this whole argument misses the point. Again, strafing runs aren't the only possible thing you can do while flying. They are one of many techniques, and your rules do not offer any benefits for strafing that do not already exist, they offer penalties for not strafing.
No, I'm running into shallow design. You can absolutely have counterplay in a game where a dragon is doing strafing runs, the problem is that 5E gives no mechanical assistance to many classes in that regard. Something for martials like, say,
a Felling Strike of some sort.
Two things.
1) If you can hit the dragon, then the strafing run was kind of pointless. The point of the strafe was to avoid being hit. If you can still be seen and hit after your strafing run, then you did something wrong.
2) Instead of forcing dragons to do strafing runs or they fall out of the sky... um... why not just have the Topple Weapon Mastery which is essentially that felling strike ability? I mean, an ability to knock the target prone with an attack is better than that feat. Also, it was a feat, so just design a feat!
But strafing runs aren't just for mechanical benefit as keeping a
mechanical limitation on the dragon: If you are going to fly, you have a limited arc in which you can attack.
@tetrasodium 's list goes into that. If a dragon is going to line up a strafing run, they only have so many options and spreading out limits damage. If you can just do whatever with flight, then there isn't that need to line things up, to give players a chance to react and limit damage, etc.
Do dragons need nerfed? Because that is what you are talking about here. You are talking about reducing the mechanical abilities of dragons. And this concept of "lining up" sounds to me like "take a turn to..." which means that the dragon is going to have entire turns of doing nothing. Monsters can't afford to waste turns like this. Not for normal combats. Maybe for a cinematic combat where the dragon is 10 levels higher than the party and it can survive that sort of showboating,
Really, it sounds to me like you just want an entirely different combat engine, which, I mean, good for you, but redesigning flight as a gateway into an entirely new combat system just seems like a lot of homebrew for little benefit for those of us who are quite happy with how flight is working.
I mean, that's great because those things are not mutually exclusive. I would say that better-designed monsters would have different flight capabilities and that would show in how they moved.
And I disagree, because every monster having unique movement mechanics sounds like a DMs nightmare.
I mean, we do, though. Certain animals can trample while others can't. Certain animals can climb, pounce, etc, while others can't. Those differences are generally just easier to model compared to flight, but they definitely exist in there.
And some creatures can flyby and others can't, and some can hover and others can't. So we have the same differences in flight. But you want more. If it was just giving dragons the ability to dash while they use their breath weapon, I'd be all for that. But you are talking about a rule set change, not giving them an ability like pounce or goring charge which is literally "move X ft, then make a special attack"
Also, horses and elephants can both trample. Do they both move in the same manner?