You need to realize that without the 6, every single one of these villains most likely had to deal with each other and Celestia, either to attain power or to defend their conquered Equestria.

So imagine things like Tyrek emerging during The War, with a battle to decide wether that timeline goes on, or deviates into The Tyrant; imagine The Night, and then having Nightmare face against Chrysalis; or a desperate Celestia turning to Discord for help, letting The Chaos act like a hard reset for any other one of the timelines.