As Shanks has no DF, one of the Yami fruits main powers - to nerf DF users significantly - will be of no use. If Blackbeard, despite this, manages to beat Shanks, no one bar prime Luffy will be able to stop him. Any admiral at this point would be stomped by Blackbeard.
And they think post Blackbeard, Akainu will appear as the EOS villain and give Luffy a fight, who overpowered both Kaido and prime Blackbeard at huge disadvantage.