I like to approach D&D campaigns with an eye for realism in the setting and NPC behavior. So why doesn't the big bad guy just tear apart the player characters when he is ten levels higher than them, rather than let them level up?
The classic solution here is the Rick Hunter/Roy Fokker or Luke/Obi-Wan dynamic, where you introduce a set of well-known higher level heroes that are renowned in the land. These great heroes are occupying/concerning the Big Bad Guy, while his Evil Minion deals with the party. The players are emulating these heroes of the prior generation and aspire to be like them.
At some point the Big Bad Guy kills the aspirational NPC heroes, which puts the land into despair and serves as warning that he's a bad mo-fo. Now it is up to the PCs to succeed where the prior generation failed.