All Run Spoilers ahead:

That's not really how timelines work though. All timelines exist at all times, because they are timelines.

The reason why Sans is so sure it'll all be reset is because, statistically, it will be reset. It's not a matter of new timelines appearing, that this one will jump into another one.

That being said, Sans is not able to see himself in other timelines, as fanon sometimes depicts. Rather, he is aware of the timelines, and is aware of an anomaly affecting and changing it, causing the deaths of everyone he knows. There are certain traces of "determination" found within timelines, and this can pinpoint exactly who is doing this.

At first, Sans figured that the best thing that the anomaly needs is some nice food, drink, friends. And while this will help in some timelines, this isn't the case in others. Nay, he doesn't grasp the concept of multiple players of the game. If he was able to know your plans, he would have killed you on the spot.

However, there is a reason he waits until the last second to fight you. He always has faith in you. As long as you spare just one thing, as long as you let one SOUL survive, you have avoided LV 20, and have avoided ending the world. But once Mettaton NEO is down, he is absolutely sure of your intentions.

Let's put this into perspective. Sans, deep down, wants you to be a good person. Accounting this, and the various other humans who had the power to reset, as well as Flowey's abilities (we know he attempted every single possible timeline just out of boredom), Sans is probably spending his time hoping things go the best way they can. And he will keep that hope. Because, even if one monster is spared, even if it's just Aaron or Vegetoid or something, there's still hope for the Underground to be rebuilt. At least then, the local townspeople who never fought you survive. They can rebuild that.

tl;dr: Sans will only fight you when it's absolutely necessary. Timelines are just researched; he is aware of them. It's not that he can see into them, he just knows what possibilities are possible.