In past editions dragons were a lot slower. With its 4 legendary actions it can move an additional 300ft (flight) or 120ft (water/ground) per round. Plus it automatically succeeds at a save if it wants and still has points left.
How long are bow ranges in 5E?
Personally I have no problem with that as I believe the players should work for their kills and that includes preventing the enemy from fleeing. But I do not believe that WotC suddenly sees it that way, too. So rather I wonder if WotC considers fleeing monsters a "valid" event in the game or if they just assume that everyone (excluding PCs) fights to the death.
Given that previous-edition dragons could already fly far faster than PCs could generally pursue them, this dragon's additional speed is merely icing on the cake.
And given that the dragon's write-up specifically says that an overmatched black dragon will do anything short of accepting subjugation to survive, it appears that WotC not only consider the possibility of escape, they're actually recommending it.
Well, at least in 3rd, your arcane caster would certainly have time to throw a long range spell or even two at a fleeing dragon and your archer would certainly manage a full volley. If the dragon was wounded enough to be fleeing, these could finish him off.
If you're lucky, maybe - but that dragon's out to 600 feet in the first round at a full flying run. That's out in the fifth range increment for a longbow, and at the extremes of Long spell range, even assuming there's no terrain to duck behind.
The 3rd edition dragon flies 2X200 ft a round right? 2x250 when it gets Gargantuan. Long range is 400 ft+40 ft./level (plus those measly 30 ft per round that you will be moving before casting).
But then it must move in a straight line, so no using terrain to hide behind. Still, pretty far out.An adult black dragon in 3.5e has a Flying speed of 150 ft, and a creature taking the Run action can move four times its speed, total 600 feet.
Well, I'm not sure the melee fighters are really all that happy fighting the dragon either, as it just stays out of reach and breathes them to death and/or tail slaps them every which way.
It does mean those spellcasters may want some decent options that don't rely on saves, though. Scorching Ray and Magic Missile style spells to the rescue.