As for your assessment based on that single line("The image disappears when struck by an opponent unless you cause the illusion to react appropriately.") .... it's wrong.
My "assessment" was a direct read from the rules.
If you've decided that the rules, as written, are wrong, then there's no sense in a discussion of the rules. I'll presume that that isn't where you wanted to go with that.
How many times do you normally have to poke a stone wall before it moves, bleeds, flinches or cries out?
Looking at the pebble trick: How do people in the real world react to someone who's always throwing pebbles at everything? Who in their right mind does that?
If someone is maintaining the illusion and you throw a pebble at it, they can let the real pebble go through while adding a real pebble to the illusion. So what our persistent pebble thrower sees is...
"The pebble hits the wall and falls, pretty much the way you'd expect."
As long as the pebble stays in the area, it works. For the lower level illusions that can only do one object, creature or force, it has to stay in contact with the wall to be incorporated into the illusion, which may look odd, but hey, it's a low level spell.
That brings up the one element that's always been missing from the rules: How does a caster makes the illusion "react appropriately"? How do you, as a DM, adjudicate that?
Someone hits the monster, it needs to bleed on cue, not too early, not too late. Timing is critical or the illusion loses credibility.
This, I think, is a crucial piece of the puzzle. The caster certainly has to be in a position to see the pebble hit the wall, the sword hit the monster. That means that they probably have to be present, and thus vulnerable to attack.
I used to use Reflex saves for that kind of timing issue. Some I know have used Spellcraft. But there's nothing actually written down, and I've watched over the years as people came up with more and more excuses to nerf Illusions.
Now, if the pebble-throwing and poking of everything is only in response to an Illusion already detected, we're talking about two completely different things. You're trying to fight a battle that's already lost.
To address your original concern: How/why does your party (or maybe your DM) greet everything they see with Detect Magic? How do they have it always available and running?
I used to play with a DM who used Detect Magic as the excuse for his monsters to spot every PC sentry on watch automatically. Somehow they always knew when they were approaching a guard post so they knew when to cast it.
Like I said, I used to play with that DM. I don't any more. Unless a monster has the special ability "Telepathy with DM", it shouldn't be given to them.
Do the critters in your game have Telepathy with DM? Or the automatic ability "Detect Impending Encounter"? Do the PCs? I doubt it.
From the sound of your original complaint, somebody isn't playing by the rules. Whether it's throwing pebbles at everything, throwing spells constantly, or poking everything they see with a stick, they'll eventually run out of spells, or people will run out of patience with them. Most people don't like getting poked, repeatedly, every time Mr. Paranoid shows up.
So if you're the DM, don't limit yourself to describing areas only when encounters are about to begin. If the players are picking up on your illusions because they're able to see which scenes or areas are important, based on the fact that they're drawn out on the map or described by the DM, then the DM is doing something wrong.
If you're a player and every opponent comes into an area with Detect Magic up and running already, then find another DM. Yours cheats.