To be honest Usyk has not got anything to lose. Huge money fight for him and even if he is defeated he will still be regarded as one of the best cruiserweights of all time. I do think Fury is the main draw in that fight as I'm not sure how big Usyk's profile is stateside.
All the politics in boxing and belts have made it a complete circus. If it wasn't bad enough having 4 main boxing organisations each regarded as having legitimate world champions in each weight division we now have interim, regular and super champions. You now have a 'World Champion' that in all truth may not even be top 10 in their weight classification. If their promoter thinks their mandatory challenger is a genuine threat they can vacate the title and seek an easier opponent for another belt. They very rarely do unifications as its better having a stacked card of so called champions. The tv organisations are the same and very rarely work together as they don't want their asset losing their world champion status.
As much as I thought it was a barbaric sport initially I've started watching a fair bit of mma. Yes, there's different promotions but it's widely regarded that the best fighters are in the UFC. They have one belt to fight for, people can't duck one another and the best fights are continually made. No Spence-Crawford issues there.