DND_Reborn
The High Aldwin
Armour Class works against combat spells, Saving Throws too. The Shield spell would be one of those specific effects that I mentioned in the post you quoted, Readied actions aren't.
Of course there are things like Defensive Duelist, a reaction further increasing your chance of being misses (i.e. interrupting the attack). You fail to realize that you are still casting the spell, but your efforts are ruined by another's action.
Consider this, your target is standing in a doorway. He readied his action to move behind the door (no longer in sight so no longer a target for your spell) as soon as you begin to cast a spell. You start your spell action, his reaction interrupts you, and is now no longer a valid target--you have to choose another.
Since you ignored the rest of my post about the time necessary to actually cast a spell, you are fine with accepting the over simplified version in 5E. Many people aren't. It makes no sense for magic to be faster than an eye blink. Take Burning Hands for instance, how long does it take you to put your hands together with fingers spread wide? About the same amount of time it would take a fighter with readied weapon to swing at you.
To say "readied actions aren't" is your interpretation. A readied action interrupts on another's turn. The precise wording of the triggering event is vital. If the triggering event is a caster is casting a spell, our interpretation is the caster can be stopped. If the triggering event was a caster has cast a spell, then you are acting as soon as he is finished.
Remember, although D&D uses a turn-based system, all of your actions must be assumed to happen over the course of the 6-second round. You are not going to move, attack, move, or whatever in an instant of time. A spell caster might take a moment or two in order to get out components, a second to make a hand motion or something, all while reciting the verbal component. I'm not saying that has to take the entire 6 seconds, after all a caster can still move their speed as well. All these actions happen at the same time everything else is going on.
Also, to say you can stop a spell with Counterspell as a reaction, but not use your reaction to take a readied action to interrupt the same spell, is illogical.