>>3791033
It's quite simple. Just make the big bad mechanically challenging to defeat and impress upon the player that they need to gather strength and allies before they oppose it directly. Doing a sidequest to delve into some forgotten dungeon isn't you getting distracted, it's acquiring valuable artifacts to help you slay the big bad's minions. Doing a sidequest to help the ruler of a local town isn't getting distracted, because now he's in your debt and will send a contingent of soldiers to distract the big bad's army. Doing a sidequest to steal a jewel for the thieves means their top locksmith will help you bypass the many traps and doors in the big bad's lair. Etc etc etc. Of course player agency is important and they shouldn't be forced to do all or any sidequest either. But to maintain the fantasy beelining through the main quest right to the end without touching any side content should be supremely difficult and require great skill and possibly metaknowledge (or just beating your head against the brick wall until it cracks).