You need to meet more 15-30 year olds. I'm part of that demographic, and many of my friends view Spotify as the first stop and Youtube as a backup if Spotify doesn't have the desired content
There's a lot of variety out there, but you're right, YouTube is the backstop.
On the house party subject, my roommates and I in college through roughly weekly parties, ranging from a few people to several hundred people, and we defaulted to Spotify (but used lots of other things as well). We could have definitely been an outlier, though, as I agree lots of people default to YouTube there.
Very occasionally he'll listen to the music I have on my home server.
Never any of the other free services.
So, I'd agree with the GP, but the demo is much younger than 15-30.