The missing five years would be the more interesting (spoilers)
1. Overall, I like this film but the ending was a little disappointing - and not because the two main characters did not stay together. Rather, their relationship was not really tested before it was over. We know they had different aspirations and that prevented their staying together all the time. But they said they loved each other at the time Mia got the role that required her to be in Paris. So what had happened in the next five years that made them leave each other for good? Simply saying that the film did not have to tell the viewer everything is not really an answer, since knowing what had happened would be crucial in understanding the two characters - both barely developed in the film. Had they never contacted each other at all in five years even by an occasional email, and not even kept track of what the other was doing via the Internet - so that at the end Mia and her husband wandered into that place without even aware that it was Sebastian's club? Also, at the end Mia had a husband and a kid. So what was her husband's occupation, so that Mia's continuous absence from home was not an issue with him, and they had even been able to have a child?
2. I also do not quite like the film's cleansed view of Hollywood - and of the entertainment industry in general. The viewers are supposed to care for the relationship between Sebastian and Mia, while we all know that in real life people in the industry change partners faster than changing clothes - and fairly often just as publicity stunts. In the film, the only problem faced by Mia at the beginning was that her "talent" was not recognized. After the success of the Paris film, her career took off and in five years she became famous. "Talent" was also all that filmmakers looked for in the choice of casting.
Look, I am not suggesting that the film should show Mia sleeping with directors in order to get roles or back-stabbing other actresses to advance her own career. Still, it was remarkable that in a film about Hollywood and the entertainment world, not one single character showed any bad behaviors. For Mia and Sebastian, I can understand that we don't want to make the characters unlikable. But in the film, not a single character in the movie industry or in the bands and clubs where Sebastian participated was shown to use drugs, get arrested for DUI, arrested for assaulting police officers or riotous behavior. There were also no hints of easy sexuality or infidelity of even one character. Apparently, in five years Mia had achieved remarkable success but was totally unaffected by these less savory aspects of the movie industry. This La La Land depicted was not like any entertainment world I know, and that I think might have prevented inclusion of elements that could have been interesting.