It is declared before the attack is resolved, and therefore before it is described as successful or not, so no retcon.
This is not correct. Here is the spell description (from the Basic PDF):
Shield
1st-level abjuration
Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell
Range: Self
Components: V, S
Duration: 1 round
An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile.
The trigger is
being hit. And then the AC bonus has the potential to
undo that hit. Mechanically it is just the same as a 4e D&D immediate interrupt effect. And is a "retcon".
Except that this paradigm is specific to 4e and matches nothing in fiction of the genre, where at least some magic is used.
First, as
@Nefermandias has posted, it's not specific to 4e. As well as the passage from 5e that Nefermandias posted, there is the fact that 5e has "psychic" damage. What is that, but a weakening of resolve and mental fortitude? And the line of continuity from Gygax's description of hit points in the AD&D PHB and DMG through to 4e's treatment is pretty clear.
Second, even suppose that it was 4e specific (and it can certainly be contrasted with other RPGs' systems for resolving physical attacks, such as RQ and RM), that wouldn't show that it's incoherent. It's completely coherent, as the quotes I posted show:
healing is the restoration of hit points, which is to say the restoration of (inter alia) resolve.
Third, there is ample fantasy fiction (and adventure fiction more generally) in which characters are able to overcome seeming defeat through resolve. I already posted an example from LotR: Gandalf lifting the hearts of the defenders of Minas Tirith. Faramir and Aragorn likewise lift the hearts of their friends and those under their command. It's a staple of fight-based films (boxing, martial arts, etc). In heroic military fiction, commanders are able to restore order and inspire their troops to reform ranks and push onward.
It's a great virtue of 4e D&D that it perfectly captures these romantic tropes. It even has CHA (Galahad) as well as STR (Lancelot) paladins!
EDIT: Here is the passage about Gandalf (pp 855-56 of my one-volume edition of LotR):
So it was that Gandalf took command of the last defence of the City of Gondor. Wherever he came men's hearts would lift again, and the winged shadows pass from memory. Tirelessly he strode from Citadel to Gate, from north to south about the wall; and with him went the Prince of Dol Amroth in his shining mail. . . .
And yet - when they had gone, the shadows closed on men again, and their hears went cold, and the valour of Gondor withered into ash.
This is not a story about the supernatural, or D&D-style "healing magic". It is a story about hope, and courage, and inspiration. We see the same in relation to Faramir (p 840):
"Faramir! The Lord Faramir! It is his call!' cried Beregond. "Brave heart! But how can he win to the Gate, if these foul hell-hawks have other weapons than fear. But look! They hold on. They will make the Gate. No! the horses are running mad. Look! The mean are thrown; they are running on foot. No, one is still up, but he rides back to the others. That will be the Captain: he can master both beasts and men. . . ."
Faramir is not a magician (which is the irony in the accusation that Denethor, who does use the magic of the Palantir, levels at him). He is an inspiring leader, whose men love him because of that.
This is the fiction that 4e's approach to healing emulates.