If Usyk loses, nobody will fight him anymore. Dubois can fight everybody. He can make good money and decent names in his resume. Hrgovic vs Dubois is good fight, but only for one belt. Nobody deserves to be unified yet.
If Dubois somehow beats Usyk which would be a massive upset in my opinion, I don't think Warren would risk it with Hrgovic. He'll try and make Fury/Dubois that keeps it in house and also generates more money. Even if they threaten to strip Dubois I think they'd rather cash out Dubois against Fury than face Hrgovic.