Interesting explanation could be found (lore-wise) in a quote from Sun Tzu:

\"Water shapes its course according to the nature of the ground over which it flows; the soldier works out his victory in relation to the foe whom he is facing. Therefore, just as water retains no constant shape, so in warfare there are no constant conditions.\"

This points out the fact that, over time, in order to be victorious, the soldier/army must change their tactics to meet the new forces that rise to oppose them.

In regards to Destiny 2 - this can be seen in the new sub-classes as an \'evolution\' of our weapons in the face of the new foe (Ghaul and the lack of Light). Where we had, for the most part, unfettered access to the Traveler and Light in Destiny, when we were pitted against Atheon, the Fallen Houses, and even the Hive\'s pantheon, in the current time, we are rendered powerless. Which brings to mind another quote from Sun Tzu:

\"All warfare is based on deception. Hence, when we are able to attack, we must seem unable; when using our forces, we must appear inactive; when we are near, we must make the enemy believe we are far away; when far away, we must make him believe we are near.\"

Followed by the advice of how to best overcome any foe:

\"If your enemy is secure at all points, be prepared for him. If he is in superior strength, evade him. If your opponent is temperamental, seek to irritate him. Pretend to be weak, that he may grow arrogant. If he is taking his ease, give him no rest. If his forces are united, separate them. If sovereign and subject are in accord, put division between them. Attack him where he is unprepared, appear where you are not expected.\"

94: Lore Dive (Defender)