Well, Mr. Sinister enjoys Hell. He calls it the best thing ever. Plus he has evaded the Third Street Girls and is running his own sadistic brothel of female demons, whom are also past Saints Row characters such as Lin, Kiki DeWynter, hell even Kia (Cyrus pays big money for this).
Yeah, that's it.
I haven't even written out a full proper story on the Third Street Girls. But what can I say.
Sentient Jack's most likely canon fate is that he is alive. Heck, Mr. Sinister may try and tempt him to replace the Boss to marry Jezebel as part of a dominance in Hell. But Sentient knows better than to trust his dark sadistic master.