There is a similar story about a red dye that was derived from a Mexican insect. For 3 centuries Spain had a monopoly on the dye and the secret of it's origin, which also was associated with royalty. Also, the kings of Rome were long dead at that point - wealthy Roman senators used the color though and after Rome fell, the color's significance was remembered and kept for nobles and royalty alike.
Yes, a different color should be used to associate royalty because it is not right that they killed so many creatures simply to get the color purple. It is a disgrace that they ended the lives of so many sea creatures. They should have been happy with more natural colors.
A lot of things that we correlate with important things have stories that date far back in history. And many of these stories are probably just as bad, if not worse than the crushing of mollusk shells. It doesn't mean we have to change the significance of these relationships, especially if the way of the past is in the past.
No, purple should still be the color used to associate with royalty, because this is what was historically done. Continuing to use the color to associate it with royalty is a fun way to remember history. There is no reason to choose another color because purple is a rich, deep color that stands out.