I'm the kind of person that thinks looking in the past to generate something new for the future as being a strong part of fashion, but I never view it as just nostalgia. And nothing is ever "new", it's just recontextualised and shifted but very few possess the vernacular to know how to do that without pandering to the whole "nostalgia" vibes. The word itself has lost a lot of it's meaning and has become so sugar coated to make it palatable that when fashion is touched by it nowadays, it feels saccharine and one dimensional (and this is what the Y2K trend feels like). A key part of nostalgia is also regret and pain alongside it's pleasures, but that is oft forgotten. It's a very idiosyncratic and visceral emotion.
The only thing I really feel nostalgia for in fashion is actually niches. The lack of having to cater to everything and the kitchen sink, which has gone by the wayside nowadays it's pretty depressing. Trends were about a common thread that happened to have occurred among the collections or a full cultural shift (like 80s maximalism to 90s minimalism), and reviewers were sometimes critical of it because it was like "really again?" whereas now trends are produced and followed massively, critics included. Trends paired with nostalgia are used to make the followers even more mindless because they've been fooled into thinking being a part of it is unique and individualistic when the reality is, they all look like pathetic carbon copies of each other. A small percentage are truly unique in fashion now, because few know what that really is.
Creatively and conceptually, nostalgia is a fantastic thing on a personal level. Used en masse, it's pandering and emotionally manipulative.