That's just a huge waste of time for players and nobody wants to deal with switching weapons based on content, or grinding an old weapon because it's better for that kind of content - would you? Every other piece of gear in the game becomes worthless when a new expansion comes out; the fact that you can trade in the zodiac weapon to get a head start on the anima is a great nod to the people who invested the time.

The point of zodiac and anima weapons was to have a good weapon that you continued to evolve over the expansion's lifetime. If you invested time in that weapon, you didn't need to farm currency, or raid, or get a crafted piece; your relic weapon stays with you. It sucks that all your hard work got turned into glamour, but so did every other piece of gear from that time. I can't fathom many people who would want to do both grinds.