Sebastian is, diagetically, mourning for a lost world of jazz which the audience knows is never coming back
But Mia’s world is the one that was about to vanish. The Hollywood in which she lived no longer exists. At the time people sensed that change was coming, but no one knew how fast (thanks to COVID) it would happen.
The movie is, ironically and accidentally, Hollywood’s perfect eulogy for itself. Sort of like how Gigi Is the perfect eulogy for MGM.