Love him or hate him Sinatraa has been grinding, he's co-streamed almost every game solo and is always providing good commentary from his perspective.
He's actually been the most consistent co-streamer this challengers and doesn't skip games like Tarik and others.
Tarik especially is lazy AF and puts zero effort in analyzing the game unlike Sinatraa.
I think it's pretty obvious that Sinatraa deserves Co-streaming rights for the next Masters.
Riot also cleared him to play, so it should not be an issue for him to get the co-streaming rights.