Swarmkeeper
Hero
No. A reaction is not instantaneous. And there is no instantaneous casting time. If there were, the casting time rules and/or the reaction description would say so. You are still conflating casting time and duration.Since shield is cast "when you are targeted by a magic missile" and magic missile has an instantaneous duration, both shield and counterspell can have an instantaneous casting time.
Do you agree?
PHB pg 202: Some spells can be cast as reactions. These spells take a fraction of a second to bring about and are cast in response to some event. If a spell can be cast as a reaction, the spell description tells you exactly when you can do so.
This point we can agree on, even if we arrive at it from different angles. Indeed, even though it is from Dungeon World, I really take to heart the Sly Flourish advice that the DM should absolutely "be a fan of the characters".One more point about the last extract, it shows that the game has a tendancy to be generous towards the players in its interpretation, because it's not fun to waste a ray or a missile on an enemy which would already be dead without it. I know some people will again say that "5e is easy mode", but it's also "5e recognises that the intention of a game is for players to have fun playing it".
Finally, I'd just like to say that I am most pleased that both you and @Maxperson reacted favorably to a post of mine and a post of @James Gasik - I feel like there we're finding some common ground here!