Why the netcode issue exists, and why it cannot be fixed.

[whilke]

With the way XI's network performance has degraded over the years, it's likely that the development team which created it was disbanded soon after the game's release. Which would mean that XI can not be considered as experience gained for XIV development. Maybe someone could check the credits for both games, though, since I'm too lazy to do that!

The same engineer has not been working on the unreal netcode for 15 years (with how the game industry operates). They just hire smart people, and build upon what they've done in the past.

FFXIV looks to have decided to do neither of those things.