On the one hand, with "Shield being an interrupt" increasing your AC by 4, meaning you don't know whether it was needed before the DM actually hits (or is the trigger "it hits", rather than "you are attacked"?), I can see how it's a gamble whether to use it. Which is sort of semi-tactical and kind of fun, and keeps the power down.
But with clever use of "reaction"'s trigger in 5e, you could react to getting swung at, so it would amount to an interrupt with the trigger "an enemy attacks you", so same as Shield. In a sense it's realistic this way because before an enemy actually lands his blow, you don't know whether your ring of protection is enough or even if Shield would help. So it kinda works. If removing interrupts from the game entirely speeds up combat, it makes it slightly more difficult for players but the risk is manageable.
Say, if AoOs are special types of "reactions", so the trigger would be "when the enemy enters your square?" Because logically, if you react to an enemy leaving a threatened square, and you don't have reach, how can you hit him? Does not compute. The reaction trigger would have to be "the enemy starts to run away"...which while mechanically in the end, identical...stretches believability.
I guess if AoOs are out in Core, then this is moot. I'd be fine with that. But you should have a way to specify a specific reaction to occur...like an enemy tries to rush past me, grab him, trip him, hit him with my sword, whatever. But that would open up basically all types of abuse too. Essentially everyone would have two attacks per round, on different turns.