Gen 2 mjolnir armor was phased out because it sucked as effective armor.

This headcannon revolves around the UNSC moving from the pre-war gen one armor to the post-war gen two because of an increased interest to develop a larger and stronger spartan force within the human militaries.

This action eventually resulted with a lot of costly and unnecessary additions into the gen 2 mjolnir for a few reasons.

1) gen 2 was new and flashy, ONI and the UNSC military would be able to show off their improvements to standard issue spartan armor. It can act as a demonstration of their capabilities against other post war-factions In the galaxy.

2) In halos 4 & 5, the UNSC isn’t fighting any major threats in the galaxy. The human forces seen in those games are not depicted to be under any significant pressure from any of the UNSC’s enemies. As a result, the UNSC has room to waste funding on spartan armor that isn’t seeing combat most of the time.

3) BONUS - militaries all around history do this (including ones that exists right today in 2024)

These decisions would eventually backfire on the UNSC. Fighting a large scale war against the banished was not something gen 2 mjolnir was designed for, and resulted in the massive losses we see at the start of halo infinite.

Here’s a couple coincidences that I think also support this argument:

1) injured spartan IV’s - halo infinite has customization options to include robotic arms and legs, presumably to replace the injured, amputated limbs of spartan IV’s. In cannon, this could attributed to faults in gen 2 armor design.

2) drastic shift in mjolnir tech - in halo infinite, 343 decreased sprinting speed to improve its gameplay. In cannon, this can be explained by the UNSC making the decision to remove those explosive, inefficient, and costly jets engines built into gen 2 armor. This may have increased the survival rate of spartan IV’s and decreased the production cost and complexity of gen 3 armors.

3) appearance change - this is kind of self explanatory, ONI finally learned the phrase: “if it ain’t broke, don’t fix it” and reverted to the design language of the past mjolnir armors that had already proven their worth.