If people can't discern quality, then I'm not sure it matters.
Lux to contemporary: Are my all my Guccis better than DH's newest MJ, yes.
Lux fashion house to leather goods specialist: My Chanel have held up well but sadly can't be spad, my H can. That may impact my usage and longevity.
It also depends on what you mean by quality. My Paul Smith bag (let's say contemporary) is almost 20 years old and I throw around that bag, the leather is thick and indestructible, it's not as good quality as my
Gucci Secret or Hermes box Kelly, which if I used it the same way wouldld look like train wrecks. Even lux to lux, durability and fine-ness are two different things. I'm not going to chuck around a Gucci python in the same way as my Hermes clemence leather bag, even though they may have been around he same price when bought.
Across a single lux/contemporary brand: Are all of my Guccis equal quality across decades, ranges and price-points. yes, and no, some are built for durability, some for fine quality and 'preciousness', do they service all of them whether made 40 years ago (last repair) or still under yearly guarantee, yes. I also know that one reason she bought more and more Furla (Italian) and Le Tanneur (French) was because they prided themselves on aftercare (at least in France) whereas she was fed-up with buying very expensive bags, Chanel, Valentino and Dolce Gahanna included, and not be able to get them repaired.