What would that movie look like, though? Who would the non-Vader characters be and what would they be doing other than getting killed?
You would still have Palpatine, Obi-Wan, Yoda, etc. plus whatever new characters get introduced. This is just a high-level overall plot summary. There’s plenty of other subplots that could exist simultaneously, but yes, quite a few people will likely get killed (that is just the nature of a story that is supposed to tell of the tragic rise of Darth Vader and a tyrannical galactic Empire). Characters that have been introduced in the first couple of films (and that we are now invested in) meeting their ends at the hands of a fallen Anakin Skywalker is the whole point. This is supposed to be a tragedy after all.