They came in from other regions bringing supplies with them. They're raiding the countryside far and wide because the adventurers hired to stop them decided to take a quick vacay. They're eating the villagers. They're eating whatever monsters ever eat in a dungeon. What throws verisimilitude out the window is that that a group can repeatedly just have a fight or two, go away for a week and expect to fight the same level of challenge because the orcs are just going to sit around doing absolutely nothing.
Seems that the adventure has changed completely. From completing a dungeon delve to stopping the orcs from pillaging the countryside and eating villagers.
Which is doable, but is more work for the DM. You have to come up with new monsters, engaging encounters, ask yourself how these monsters were brought in, what the consequences on the environment are (hey, the spider-rider Arachnid clan are involved, maybe there should be a bunch of spider webs as foreshadowing).
Which ties neatly into the original point: if the DM has to do a bunch of extra work whenever the players engage in a 5MWD, than it’s not a DM problem, it’s just a problem.
Do you ever seriously go into depth about what the monsters eat? The ecology of monster infested regions or caves has never been a strong point of D&D so maybe the orcs are just eating all those strawmen being thrown out left and right.
All the time. Both as a player and as a DM. If a dungeon is cut off from any reliable source of food and water (say, a wizard’s demi-plane), the monsters in it are likely going to be Constructs and Undead that don’t need to eat.
I certainly don’t use ninja divisions of orcs: large quantitities of orcs would leave pretty obvious marks on terrain as they go through.
The purpose of this verisimilitude is that it allows players to better interact with the environment. If I’m going into a wizard’s abandoned demi-plane, I should probably avoid spells that target a creature’s mind. The signs of a large hostile force means that it may be an encounter that can be avoided.