I own Missoni orange label as well as M Missoni. I have been buying and wearing the brand for probably over 15 years and I feel that the Target line in no way cheapens the brand.
I also own BV, Chanel and Hermes and would not care at all if they did a line for Target or Walmart either. If the lines were exactly the same as the more expensive line, then I would question how "luxurious" that brand was to begin with. Since almost all luxury brands have a more affordable line these days (and this is nothing new it has been going on for years), I don't understand the objection to some but not others. Didn't Karl Lagerfeld do a line for Macy's and earlier for H & M? I don't think that made most Chanel fans stop buying the brand. Ralph Lauren, Donna Karan, Marc Jacobs,
Prada, Halston, Valentino and many more brands have a lower price line. I don't see any difference here.
Also, the lauch of the Target line, made me go through my closet and start wearing the Missoni pieces I already had. And a relative went out on a Missoni hunt and bought a few (main line) things after seeing the Target ads. So, imo, the Target line may have prompted Missoni customers to want more of the main line items also.