A potential solution to item level syncing & power creep:

[Azuri]

Not all 345 pieces are crit > det. If you build a full 345 set versus a synced crit > det set you won't have similar substats. For the example chest piece specifically, yes you're right on that but there's a difference between a full 345 set with melds and synced pieces.

Even then this affects content outside of ultimates.

That doesn't matter. Difference between individual substats accounts for sub-1% final dps variation. Your substat totals are what matters for this discussion. And you are willfully ignoring 32 substat you are going to have from materia in your argument.