The sort of classical jazz that Gosling's character prefers is definitely a niche product now, compared to its heyday.Isn't the jazz club part of Gosling's motive to help jazz stay relevant?
I think the criticism comes from the scene where Gosling explains to Emma Stone's character why jazz is great and the idea that it is dying, as well as Gosling showing disgust for John Legend's sequencer. It's not people mistaking the opening of one jazz club as saving the entirety of jazz.
People are conflating the POV of Gosling's character with the film's. The film never suggests that Legend's character and the music he makes is bad (among other things, it was one of the three they submitted for Oscar consideration, so clearly it's not supposed to be) -- it's just not the kind of music that Gosling wants to make. At the end of the movie, he's opened his club and found his niche.