So the transition from emergent genre to legacy genre is the very threshold at which retro becomes possible, in all its permutations (commemorative releases, oral histories, reissues, revivals, homages etc).
In between though there must be a stage of reasonable duration where the genre is neither unknown nor over-familiar but is in its prime: freshly accepted, but still surprising, still growing. Perhaps that corresponds to what some call the Imperial Phase (usually with an artist not a genre: when a group or performer is at at the top of the world AND the top of their game, creatively -- just runnin' tings).
"Legacy" is a nice way of saying "old news". Which is a way of saying, "something you can't be an early adopter with, because you're too late, mate". For journalists, "legacy" also means: hard to get an assignment to write about in magazines, apart from specialist publications. ("What's the story, here? "Techno, Still Pretty Good"?!?!)
Perhaps when a genre reaches the legacy phase, it has a history behind it - but it can no longer make history. Its normalisation -- its acceptance -- is both its triumph and its downfall, because it's just part of the landscape now.
Is there a subtle difference between "legacy" and "heritage"? Legacy perhaps leaves open some kind of functionality in the present ("a living legacy"), an ongoing state of relative vitality, the mature refinement and perfection of the already-formulated. "Heritage" just seems to suggest a museum culture, a style of music to be documented, studied, memorialized preserved - propped up with reverence.
Perhaps they correspond to stages of life....
emergent / childhood-teenage
imperial / young adulthood-thirtysomething
legacy / middle age
heritage / old age