That was actually one of the main underlying plot points of one of the later SAO arcs. The story conclusion was it's more or less "okay" if it goes into the "more real" direction, i.e. if you find yourself trapped in a simulation without escape, you go crazy because it's "not real", but if you suddenly one day find that your life this entire time was a simulation, that's the norm for you, and it would also be okay for you if you exit the simulation into the "real world".
If you free yourself from any and all external motivation, you will eventually find some new goal. You'll look at the garden one day and decide to start growing veggies or keeping it free from weeds, or you'll think to write a story or find a new book to read. You do it because you "want to", intrinsic motivation, as opposed to "have to", such as finding a job to pay rent or defend your country at war, external motivation. It happens much more often then you'd think; you get fired and you don't need a source of income for a few years so you reevaluate your life, or you get an inheritance and decide you don't want to continue old job.
Completely zero motivation is still not actually zero because you have basic physiological needs.