Wouldn't change anything. I already know for a fact demons exist. And there's always gotta be a balance... Good, evil. Yin, yang type shit

It depends on what you mean by "demon." IMO demons are just spirits in darkness/ignorance / immature/aggressive/destructive/evil spirits, but not categorically different from non-demons. Given enough time a demon will get over that phase and grow into a decent spirit. So if it's somehow proven that demons are categorically different, that would change my mind about that. But it raises questions about in what way, and why, they're categorically different, why they exist, etc., and the answers to those would have further impact on my worldview.

wouldn't even flick an eyebrow
i know so many rpgs with that stuff...
already thought about it

It would change my entire world view as I don't believe in them.

It would seriously depress me.

Outside of potential changes to society that would arise from it? Not much.

I lived my whole life and ostensibly they didn't show up that day...they would have been around my whole life, so knowing they're around now doesn't really change anything

Geez... I'm not sure my ego can imagine not being on the top of the foodchain... needless to say most of the shit in my life wouldn't matter anymore :o