You can see an afterglow player a mile away, I think it would be less aesthetically pleasing to have the effect stronger (less transparent) than it currently is.

Also you should consider that there are other afterglow owners (or people working on it still) that find the visual effect in its current state visually pleasing.
FFXI does not do per pixel soft particle rendering so a more intense (less transparent) effect would make it more noticeable when the individual afterglow particles clip through the model, creating a worse looking effect.

Empyreans in general are weaker than mythics, so the crit% bonus empyrean afterglows receive somewhat offsets this.

Afterglows don't need to be better in the effects they confer, that just makes marrow/cinder/dross/scoria more expensive for people trying to make the base RME weapon.
We don't need more people competing for those rare materials at such a high volume.