Mettaton sits there, quiet and patient in the ensuing silence. He's looking at Sans and the skeleton's eyelights seem far off and distant. He's shaking a little but beyond the hand holding Mettaton lets him be. He'll answer when he answers and he'll pull away if he wants.

When Sans does finally speak, it takes the robot a moment to really process what he said.

"He doesn't exist?"

How? Was this Monster so impossibly shitty that the universe decided 'You know what? Fuck that guy and booted him from existence? Mettaton pulls himself back. Ok. Try to be reasonable about this. This is certainly more than you expected and if Sans isn't being, like, needlessly dramatic, you need to be as kind as possible.

"Ok. Well. I'm not going to lie to you, Sans, that is certainly not anywhere in the realm of what I was expecting to hear."

No point in beating around the bush here. This is a bit out of Mettaton's depth. But when has a little thing like not knowing what he's doing stopped him before?

"I'd say, though, that despite the, ah, peculiar problem of his lack of existence, it still matters. If it's hurting you, it still matters. If his lack of existence is still affecting you and apparently other people, it still matters."

Then Mettaton, suddenly, chuckles. And looks Sans dead in the eye sockets. "That's not your decision to make now is it? I don't think I am wasting my time on you; therefore, I am not."

Good luck trying to convince him otherwise, Sans. He's a pretty stubborn bot. You gotta be if you want to be good friends with Napstablook and Alphys and Chara now, too.