They cannot do anything drastic to stop Sabo once he's out the room, unless they want to disrupt the reverie and get attention attracted to them.
They were doing their best as much as the 3 admirals did their best to kill or capture Luffy, when he jumped in front of them. A Luffy who Akainu Kuzan or Kizaru could've easily blitzed and put down. What happened instead? Kizaru kicks him away into the battlefield and then he gets rescued. Kizaru aims for the key, instead of blowing off anyone's head.
And unlike the Gorosei or Imu, they wasn't restricted to not causing any commotion in the castle, to prevent the secret for getting more exposed as well as the ongoing reverie.
It would be the equivalent of a white beard fan defending whitebeard instead of acknowledging that Akainu just had to aim for the head and WB would've been GG.
It's simple plot, except Gorosei have actual reasoning this time and Sabo being a solid fighter.