In games that have significant power advancement over time, not having a BBEG can generate plot holes, in much the same way Elminster and the Avengers do.
To wit: When you have a powerful force that could have been involved, it brings up the question of why they weren't involved.
Like, this powerful, cunning, magically advanced dracolich with a massive support structure of servants just happened to manifest now, and had no impact on the world prior to the characters achieving a level high enough to defeat him? Or, with all the skullduggery the PCs have been involved with, they have never heard of this threat until just now? Really? Is that plausible?
More common in my experience is the GM who, in worldbuilding, has several different things that might be threats later in the campaign. They are not idle while the characters are young. So, the PCs will stumble over some of their works, and eventually choose which of them they intend to deal with - they choose their own BBEG.