As the title says, i believe that Marco has been the savior of the alliance until now. Let's do a small recap ever since the Beast Pirates allied themselves with the Big Mom pirates
- Marco reduced the BP+BM pirate alliance practically in half by sending BM pirates crashing in the sea(outside of BM and Perospero), so without Marco, the alliance not only would have had the current problems they are having, but this alongside fighting the almost entirety of Big Mom's crew, and judging by how much they are already struggling and overwhelmed in number, it'd have been a crushing defeat
- Zoro was still stuck on the live floor wondering how to get passed King+Queen. Without Marco then, Zoro still would have been there and at best would have exhausted his entire forces just to try to go through King and Queen. In the meanwhile, in Zoro's absence, Kaido and Big Mom would have overwhelmed the worstgen on the rooftop even easier
- Queen's virus was continuously spreading in the entire live floor for enemies and allies alike until Marco came, and thus if not for Marco stopping the virus from spreading due to his blue fire, majority if not the entire live floor would have turned into oni by now if not outright dead, including Chopper who is the only one capable to reproduce antibodies to the virus, making the alliance lose the live floor battle without King and Queen even having to do anything
- Without Marco, no one would have been able to stop King and Queen from killing Chopper(only one capable to reproduce the antibodies of Queen's virus), the samurais on the live floor or the scabbards, with Marco also taking advantage of this to weaken them in the process
So i think with this, it's pretty clear now that Marco has been carrying the alliance on his back. Without him, they would have already lost