Misty step, disadvantage for the Dragon due to foresight, and ridiculously high AC from haste and/or shield. Eldritch Knight remember? Also the Dragon has to beat their athletics check, probably not going to happen.
AC does not apply to grapple. And if grappled, he could be 100 to 120 feet above the ground in a single round. Yes, he could Misty Step 30 feet down, but then he'll be prone on the ground taking 14+D6 of damage.
Eldritch Knights get Foresight?
Or is your 17th pal Bard or Warlock or Wizard casting that on him? And why is that PC not casting that 9th level spell on himself? Your entire scenario here only really works at real high levels.
What if the Dragon casts Dispel Magic? If it works, oops, tactic lost (and Haste and/or previous Shield automatically get dispelled).
The point is, if the PCs have these types of tactics and abilities, a DM is going to give a solo Dragon decent tactics to either counter them, or to make them less effective. Heck, most DMs would never have a solo dragon fight a high level group like this in the first place. Dragons are intelligent. They'll get a group of lesser creatures to attack with them at a minimum.
Because the Fighter can shoot at it from 600 feet away if using a longbow, and a bit closer if using a crossbow, and is capable of doing 300 hit points of damage to it in the first two rounds of combat.
Why exactly is the Dragon letting the party see it from 600 feet away? Why is the DM playing a 16 Int Dragon stupid?
What level fighter is this and why is he getting 5+ attacks per round starting in round one? How is it that he hits the Dragon every time even with -5 to his advantage? How is he coming into the fight with the one action Haste spell already cast? How is that Haste spell surviving the first breath weapon?
How is he always doing this? Does the Dragon not have spells too? Can it not cast Greater Invisibility and the Fighter has no clue where it is? Can it not cast Force Cage around the fighter and maybe get a few rounds respite from the longbow of death?
The Dragon on the other hand probably isn't going to hit the fighter AT ALL with shield, foresight, and so forth. The only thing is breath weapon, and he still has 180-220 hitpoints, and the Dragon has to get line of effect against him (breath weapon can't go around full cover). Also Fighter can have Protection from Energy up as well from his Wizard/Bard/Whatever friend who is Ethereal keeping buffs running (or cast it himself as an EK).
When I fought an ancient red, by the time it actually got close to Fighter, it was already down about 400 hit points and it was basically time for it to retreat. And yes, it was using its legendary actions to move and get closer. Didn't have magic items either, with those, it would have been even more one sided.
How many buffs can this theoretical friend in the Ethereal plane keep up? And how exactly is that party member helping if he is on the Ethereal plane? What happens on the round that the Dragon casts Etherealness to get away from the PCs and finds a PC there? That PC quickly goes back to the normal plane and the Dragon then follows the PCs around ethereally for up to 8 hours, just waiting for a chance to do something nasty and unexpected.
Or, what if the Dragon figures out that someone is in its domain and casts Etherealness to find out more about such intruders?
It does not matter what abilities PCs have. If a DM is doing a good job to challenge them, the PC tactics are not going to work every single time. The DM will throw monkey wrenches in and set up unexpected twists and turns to his encounters. Or at least, a good DM will do that. He won't allow a few PC combos in the game repeatedly turn Hard encounters into Easy encounters. And he won't let the PCs determine the time and place of every battle. The dragon that went ethereal to escape comes back two weeks later ethereal while the PCs are resting in town and spies on the PCs and then comes up with a plan to take them out one or two at a time.