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L2's dungeons had a ton of dead end rooms and quite often they would have the best mobs in the dungeon or just a boss spawn point, so there weren't always a way out.
As for notices, that would only work if you're a part of a guild that's at war with another guild and you're always on the lookout for them. But quite often you might just be a party of people who're farming a room and there's another random party that comes in and wants to fight you for that room.
If you're a group of 8 people and the mobs are properly difficult, you won't have anyone to be the lookout, standing somewhere else in the dungeon. Though even if you did stand somewhere else, there's no assurance that the party that's running through the dungeon is going directly for your room, so you disengaging a valuable mob preemptively might just fuck over your ability rotations (assuming we have super long cd abilities with high value).
And even if you have a lookout and he stands right outside your room, as I've said, your room might be a dead end and the only way out is through another group. But at this point you're at low mana/health because you were engaging the mob, so any kind of pvp would be hugely disadvantages to you. And depending on how Intrepid sets up loot rights checks, if you just run out of the room and the newcomers finish out the mob (because they've got all their abilities at the ready) - you might lose your valuable loot. And if you don't run away, we come back to the "pvp is shitty for you cause you're not at full power".
None of these things were really an issue in L2 because pve was super basic, but if Ashes will indeed have complex and hard pve then there's gotta be a way to manipulate mobs in your favor, in case someone comes to take them from you. The newcomers would have the same tools too, at which point it'd become a game of chess and proper party gameplay, where the more skilled group should ultimately win.