Moving his turn earlier doesn’t necessarily mean the ending is nothing but killing Jedi. A more convincing arc could’ve been:
Set Anakin up as a hero
Anakin succumbs to the dark side
Anakin’s friends fail to pull him back to the light
I mean, there’s more to it then that. There’s also character development for Vader delving into his psyche and showing his adjustment to becoming a cyborg monster. Basically, the stuff in the 2017 Vader comics (which IMO are a masterpiece). Just thinking about a movie adapting Vader bleeding his kyber crystal and the Fortress Vader arc gets me giddy.
I don’t see how this is better then having Phantom Menace and Attack of the Clones (tho improved, obvs), then a Clone Wars movie to really show Anakin’s heroics, then Revenge of the Sith and then a Vader movie. I think that Anakin having a traumatic past and a hard time letting go of people he cares about (due to his mother’s death) is fantastic and adds dimension to his character.