....
New fanbase??
No it wouldn't.
Ngannou isn't very popular.
I wouldn't shell out 80, 90, 100 dollars to watch Ngannou get schooled in a boxing match. And neither would most mma fans. Most boxing fans would be pissed their legitimate champ would be boxing a what really is a freakshow fight when there's legit matches available. And the biggest leverage Ngannou had was the UFC HW title, which he doesn't have anymore, which would've gotten the casual watcher to pay some attention to it.
This is the worst gamble Ngannou could've made. Fury, Wilder, Anthony want to box the UFC HW champ not Francis Ngannou. The casual spectators want to see the UFC HW champ box a boxing champ not Francis Ngannou.
People don't realize most mma fighter's popularity is directly tied to the UFC promoting them. Once they leave the UFC they are quickly forgotten. Go outside and ask 70 guys and 30 gals who Francis Ngannou is. I'd be surprised if 5 people could tell you who he is without help and mentioning the UFC specifically.
Let's be real. After Conor cut that promo at the club saying he wanted to box Mayweather that's idea caught fire. Everyone and they mother was calling for that fighter. 15 minute segments on sport's talk shows dedicated to discussing the possibility of McGregor boxing Mayweather.
It's been a long time since Ngannou last fought and since he was in the ring with Fury. During that time not a peep from any major sport's show talking about the possibility of fury boxing Ngannou. I don't see sherdog creating a whole sub forum to deal with the amount of traffic that fight is bringing to the site.
Like almost all mma fighters their popularity and financial ceiling is directly tied to the ufc and not their own marketability.