Why exactly do Galvanize and E. Diagnosis overwrite each other?

[WaxSw]

Balance.

Double shield stacking could allow for players to cheat the "dance" since many of the mechanic's debuffs are applied the moment they damage the player and a shield that covers the whole damage nullifies the application of those debuffs.

It also increases the problem of double shield comp being far superior healing wise than shield + regen or double regen

[Astronis]

Double shield stacking could allow for players to cheat the "dance" since many of the mechanic's debuffs are applied the moment they damage the player and a shield that covers the whole damage nullifies the application of those debuffs.

Virtually guaranteed it's this. If you can avoid the debuffs via shields, there's all manner of mechs you can cheat.

[Daeriion_Aeradiir]

Balance.

Double shield stacking could allow for players to cheat the "dance" since many of the mechanic's debuffs are applied the moment they damage the player and a shield that covers the whole damage nullifies the application of those debuffs.

It also increases the problem of double shield comp being far superior healing wise than shield + regen or double regen

Every single debuff in Savage+ these days always applies regardless of how much damage you take, they intentionally started doing this in EW when lots of vulns were being ignored in Eden tiers due to super shielding.
If any direct mechanic even remotely dangerous in any fight's "Dance" could be ignored with shielding, you 100% bet stacking tons of party mit on it to zero it out would become mandatory since SCH can save a crit-adlo for it, but that's not the case, all debuffs that matter get applied anyway.

That being said, it is the general correct thought process. The shields don't stack because most healing in higher end content comes from preventative measures rather than reactive healing, so double shields would be stupidly OP, especially in phases like TOP P6 that are entirely mit-check based rather than healing based. Double shield healer is already insanely strong even without stacking shields because of the extra mit, preventing shield stacking is basically the one thing that merits exploring AST/WHM as the other option.