I think it's exactly that. Chris finally had some validation in life (yeah it was Barb and to us that's horrific, but for Chris is was better than a lifetime of nothing). Someone who genuinely loved him and would fuck him without being paid (although maybe only because of the dementia). This energized Chris in a way that he actually became something slightly, ever so slightly, resembling a productive human being.
In my own personal experience I am far more productive when I have a companion than when I do not. I can be a LOT more productive than Chris, obviously, and actually accomplish useful things instead of shitty kindergarten scrawls, but the change is noticeable.
Imagine if Fiona had appeared sooner -- even by just a few months. Imagine she genuinely wanted Chris long-term -- even if it was for infamy. Imagine she lived in the C-ville area so she was genuinely accessible. Imagine she became Chris' gf. I think Chris not only would never have fucked Barb, but would have had greater productivity than even the spurt he showed in that last month.
Yeah Chris turned Fiona down, but Fiona was a far away idea no different than any of the trolls in the past. Meanwhile Chris, for the first time in his entire life, had a real and tangible heartsweet right there. It's like spending your entire life starving, with people everywhere showing you pictures of food but the food never being real. Then finally, your shitty garden produces a horrible misshapen crop, but you get to taste nourishment for the first time in your life. The pictures of hypothetical food suddenly become less appealing.
It's twisted, but Chris the motherfucker was becoming a better Chris in other ways. I bet it would have worked even better with someone other than Barb, but only Barb was in a position to fill that role.
