Go bloody mad/sad. The dissatisfaction that comes from losing something you always could do and probably took for granted would be immense. And music doesn't just impact songs.. it would make shows and movies and video games suck too, since they rely so heavily on music to texture scenes and create mood.
If he has game pass arguably he has plenty to play that doesn't require dipping into Steam. I mean Silksong was just released after all...
JRPG wise Clair Obscur is still on game pass iirc, so the steam only now that's also on currently on discount would be Persona 5 Royal or Metaphor Re Fantazio.
The only reason I can think to play on Steam would be for Valve exclusives like Half Life series, Dota2 or CS:GO.
Your right