It doesn't need to be proven, lol.
The DEVs OBVIOUSLY do not want it right now. They may change their minds, if enough people (especially in the JP area) request it but that remains to be seen.
Allowing to dye every piece of gear would increase glamor options too and yet developers to not wish to do it. The game can handle it just fine and it is hardly more effort than setting a flag in the colorset.
Heck, they even lock the option behind high end raids, putting the dyable version out of reach for many people.
So yeah "moar glam options" is clearly not a sufficient argument for the DEVs to deviate from their design vision.
Got anything else?
Edit due to stupid "no more posts today" bullshit:
From a purely technical PoV you actually REDUCE glamor options overall, as you remove one unique item from the pool to replace it with the same variant from the other sex. Instead of males looking different from the females by default, they now look the same.
If you want to globally increase glamor options, you'd need to retain both versions for both sexes.