Interesting piece, even if I disagree — specially when you refer to how "others […] are obstacles to overcome". Some people are, sure, but Keith is not an antagonist as much as he is a dissenting viewpoint. He is there to tells us (and the viewers who might not be familiar with jazz) that Sebastian might just be clinging to the past, and it stops Sebastian from coming across as an unabashedly positive crusader for Ye Good Olde Days. If anything, Keith coming in marks the point in which the movie stops lionizing Seb, where it had only poked a bit of fun at his self-importance before.
"Many of La La Land’s most beautiful visuals covet isolation in a way that becomes toxic in large doses"
And yet the movie does not extol such toxic large doses. Sebastian ends up surrounded by people who appreciate the same music he does. Mia is happy with her new family and the dialogue with her husband indicates an extended network of friends. And some of the movie’s most up-beat, peppy numbers are those with the most people on screen.