Most people don’t intentionally listen to 100-year-old music, sure, but you’re underestimating how much we absorb this stuff as background music in ads, movies, TV shows etc.
Most classical music is very niche but a few pieces become cornerstones of popular culture -- think of “flight of the bumblebee” or the William Tell overture. Satie has a disproportionate number of hits. His style is exceptionally simple, distinct and timeless.
If you’re older I can understand why it seems perplexing but it’s true in my experience.
But what you said was:
Youtube is probably hugely responsible for Erik’s modern popularity.
That makes it sound like people weren’t aware of this music before YouTubers started picking it up. That’s very much not the case.
If you’re just talking about name recognition rather than the music itself, I suppose that’s possible as I don’t think his name was all that widely known. You’d have to show that it is more widely known now, though.
My guess would be that his music is familiar to quite a large fraction of people, but that most still don’t know his name.
I’m not trying to be snobby about this -- there’s plenty of catchy classical music I really like where I can only vaguely guess at the composer. I just happen to know and like Satie. It helps that a lot of his music is relatively easy to play for an amateur pianist like me.