The upside is more importantly that screenwriters, directors, and any future DC story tellers are not held back by the previously released DC content. If a writer/director decides to make a movie in a new universe they are not constrained by what happened in prior DC movies. If they decide to continue another universe they can go right ahead as well.

Matt Reeves for instance isn't hindered by the Joker movie. In the end of the Joker, it is shown that Bruce Wayne's parents are killed by a Joker inspired citizen during a riot, not a random mugging by Joe Chill. This means Reeves doesn't have to implement that aspect into how he handles his characters and story, therefore giving him more creative freedom and control over his movie.

DC' approach leads to more creative freedom which is obviously very appealing to writers and creators in the film industry. In Marvel's case, creators needed to be careful in what could happen in their movie since it would affect the whole continuity.