Or... Players could save before the final quest and reload after seeing the ending.

So! The player has battled most of the way through the story, usually to the start of the final climactic battle scene, or last epic mission, and the friendly NPCs are all excited, talking about how time is short, gotta save the world, building suspense, blah blah ...
and then the hero of the story is like:
"Whoops, I forgot to run some random errands, I'll be back in like, 2 weeks!" and peaces out.
- everybody left standing there, scratching their heads, the villains are checking their watches and trying to bum smokes.
the forces of darkness politely bide their time and wait for the player to get back.
the fate of the world hangs in the balance while the hero wanders off and does some random looting and gathers herbs or something.
THIS HAPPENS IN AMOST EVERY RPG I PLAY. DAMNEDEST THING