Even then they can simply drop large rocks on the PCs till they are dead or use trees as improvised reach weapons by simply dragging them across the ground while they fly (isn't that the kind of stunts people want in combat?). A smart villain will press his advantage for as much as he can. For a dragon this means staying out of the reach of melee characters unless the enemies have more ranged firepower than he has (not always guaranteed).
Problem is, smart enemies are often not very fun to fight, especially when the rules set prevents you from being smart yourself like 4E (focus on combat as sport, discouraging battlefield preparation and flexibility, especially in the weapons department)
That is a whole different issue.
That is Game Given actions Vs Human created actions.
If you give dragons full access to Human Created actions, of course it would be easy to be not fun. The dragon has flight, fear, strength, hardness, spells, natural weapons, and a breath weapon.
PCs cant win the arms race as they only maybe have combat abililty and maybe spells. The only advantage the PCs have is action economy as there are often more of them.
If you go the heavy Game Given action route, the dragon wins as they have more. The only way to make it fair is to weaken their actions (less spells, limit breath attack)
If you go heavy on Human action way, the dragon still wins as they have more methods to make actions. Then you have to make them stupider to be fair.
Also 4E
does not prevent you from making new actions. It does however, give you
so many Game Given actions that people
forget to create their own. So many paints, you forget that you can mix blue and red to make the missing purple.
And even outside of combat, smart flying can become "unfun" for the players, for example the dragon can fly in, let the PCs waste their spells/buff to prepare then fly away again to wait for them to run out. Or the dragon can simply flee when wounded.
Still I would take all this issues over removing the flight from dragons (make it cutscene only) because it forces the players to play smart instead of just doing their combat routine.
Removing flight is lame and gamist.
Like I said in other thread, instead of banning things, make them work.
Make dragon flight too poor and slow to allow kiting and escape.
Restrict breath weapons to one or twice a fight.
Limit the amount of ranged attack spells the dragon knows.
Then when the PCs are high enough level to all fly and deal with spells, take the training wheels off.
Adult black dragons pull victims one by one into magic darkness or controlled waters.
Adult blues are literally flying tanks as they focused on their breath attack
Adult greens focus on their draconic aspects of fear and charm.
Adult reds and whites still go in melee but with more deadly results.