The average range of most spells is simply too short to deal with this tactic. The dragon bypasses all risk before attacking by staying well beyond everyone's range. In a single turn it stops flying, falls to 50 feet above the party, casts featherfall to catch itself, breath weapon on everyone, flies sideways 80 feet. It's not 94 feet from the party. All weapons except heavy crossbows and longbows are now at long range. The majority of spells are also out of range, though there are some outliers that will still work.
Thanks for elaborating, but that wasn't really what I was asking. I meant, "Why in the world does the average party
not have multiple characters capable of attacking an enemy 600 feet away, let alone 100?" It's a glaring vulnerability if you don't at least have longbows. Spells would work against this dragon's tactics because it has to come so close to breathe on you: the Feather Fall trick will work if you catch the party by surprise, but it won't work repeatedly because the party will just ready spells.
In point of fact, the Feather Fall trick might not work even the first time. I can see two failure modes:
1.) I don't know where you got 94 feet from but let's go with that number. Since a large component of that 94 feet is horiztonal, a party member can burn part of their 30 feet of movement to get within 65-odd feet of you and then cast Hold Monster (90' range) or similar on you. If you're a dragon too young to have Legendary Resistance, this could be the end of you. In short, 94' feet isn't that far away, even against spells.
2.) Somebody Counterspells your Feather Fall. Splat.
Edited to add: Initially I thought you intended for the dragon to use this tactic repeatedly ("
Repeat every few minutes to waste party resources."), but as an opener it's fine and worth trying. The one thing I don't get is, if the dragon is invisible and catching them by surprise, why even bother with the Feather Fall trick? Why not just invisibly fly to within range, then open with breath weapon (probably gaining surprise), and fly away normally?
Edit2: I agree with your general strategic thinking though. A smart dragon can win by going cat-and-mouse. If a dragon surprises the party, grapples one party member and flies off with her, the party may inflict a lot of damage on it (100? 150?) but the dragon kills the PC and then rests for an hour to regain HP. It's now back at full HP and the party is down a member. Score one for cat-and-mouse tactics!