Certainly. In the case of the goblin/hobgoblin and the two orc caves, they are actually connected to other caves and presumably have daily contact with each other. In the case of kobolds, they do daily chores for the other lairs and so have daily contact with almost all of the other lairs.
Wait... what?
The bugbears have a little shop and pub for the other denizens of other lairs. [/quote]
again... what?
And in any case, all of the entrances of the caves are in the same little valley and you can see from one cave entrance directly into most of the others.
Have you actually
LOOKED at the map?
It's obvious that this is a very tight little ecosystem that exists in precarious balance, and if the PCs come along and upset that balance, the consequences will be felt very quickly. All of this is made explicit in the DM's text. If the PCs go into the cultist cave, the owlbear cave, or the minotaur cave, or perhaps even the gnoll cave, there will probably not be immediate consequences felt on the rest of the area. But all other caves are very connected to at least one other lair.
Umm, the only connected lairs are peopled by similar creatures - goblins with goblins by and large (since orcs are considered goblinoids at this point in time.
But, this gets back to my point. This is why I often say it's ridiculously easy for sandbox DM's to railroad. Because, that's exactly what you're doing. The players are doing something you don't like, so you pick the results that most punish their behavior, not because it's believable, or even realistic, but because you want to punish their behavior.
Let's run with your example for a moment. PC's go into a cave, wipe out some of the creatures and leave. Next cave over comes in and finishes off the job and loots the place.
How is this a failure for the PC's? They simply go next door, kill the next guy over and get the treasure (which is the lion's share of the reward) from the now weakened (since, well, 12 hours earlier they were fighting - you do account for that being all about realism don't you?) next group and they win.
Or, taking this a step further, how would this not cascade? PC's go into one cave and weaken it. Next cave over comes in and wipes them out but gets weakened in the process. So, the next cave in line comes in and wipes THEM out. So one and so forth. The PC's come back 12 hours later, mop up the survivors and get all the treasure conveniently gathered in one place. Total win.
Let's look at your other option - retreat. PC's enter cave, wipe out some of the inhabitants and leave. They come back 12 hours later and the cave has been evacuated. But, the evacuees are mostly females and young. And they leave tracks. And they're burdened by their belongings.
Why doesn't the party simply follow this large group and wipe them out considerably more easily since they aren't hiding within their nice strong lair? This plays directly INTO the hands of the 15 MAD group. They've now gathered all the inhabitants and their loot into one place, the PC's who will move faster than the evacuees (having horses and all, or, even on foot, they're still faster than burdened kobolds) can pick an ambush point wherever they like and bring 100% of their firepower to bear in that ambush, instead of having to ration things across an adventuring day.
Both of your "solutions" actually play directly into the tactics of the 15 MAD group. They actually make the 15 MAD group more effective, not less.