What are they fighting on? The moon? Not a single island that can survive a clash of this magnitude.
How does Blackbeard even go all out without getting in the rest of the Yonkous' way? The scale is too big the island sinks and Big Mom potentially drowns like she did in canon unless Shanks decides to take a dive. Kuzan can freeze the ocean, Akainu can melt the ice, Fujitora can float on land, Teach and Shanks have no footing unless they take a ride on Kaidou but honestly that's too much brainpower and teamwork to expect out of that team.
Admirals have a much higher chance of surviving an encounter like this not only thanks to their versatility and scale, but because they actually have a functioning brain that can make level headed decisions, Yonkou are a clusterfuck of power that's all, no organization, they'll just get in each other's way.