Kla Ilan or Indigo is a plant.
Plants are constantly bred to improve qualities. Wheat and rice have much larger yields today than they did 200 years ago because of improved genetive selection. The same is true of apples and even esrogim. The are far better quality today than ever.
Why should Indigo be different?
The colorfastness of this plant is probably a dramatic improvement over what it was in the time of ChaZaL.
The most powerful argument for Murex is that if this is not the true chilazon, then – since the dye is extremely similar to tchelless – Chazzal should have warned us about this fraudulent tchelless. Since they didn’t, it is clearly the real thing.