valery
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You need to find a way to exploit the Best Defense Position (BDP).
And who told you that this tactic works in 100% of cases?
But what's the point of it in the endgame?
If the map is designed for rushing, then it's very effective, but in a long game with level 30 units, breaking through defenses doesn't matter as much (though there are exceptions...).
If the map is medium-sized or even larger (assuming there are no unrealistic garrisons), then you can break through a lot with this tactic (though not everything), which means you can still rush.