If I ever create an RPG, it would be a good idea to put an invisible timer on the main quest, and if you don't complete it... nothing visible happens, but by the time you reach the final boss, he had completed his evil quest, your wifu dies, your home is burned to ground and you get the hardest battle of your life.
On the other hand, if you do only a reasonable ammount of side quests, then you battle him before he completed his goal, and you can actually get the good ending.
@apropos @Jens_Rasmussen
Yeah, that's the sad part, that if I were to do it, it would literally be the first thing everyone would hear about the game, making the mechanic meaningless.
In a fantasy setting it could be astrological. When the planets finish aligning the Demon King's final spell will start.