Destroying Darth Plagueis isn't immediate. It isn't something they can do overnight, even with their two new (quasi-willing) recruits, even with - whatever the fuck Ridis is - giving them a way to hit Plagueis from a direction he never predicted. He's old, and while the transmission of power from Master to Apprentice has never been thorough, he's the heir of the Line of Bane and has at least some benefit from that millennium the Sith have spent in the shadows, building their power.
Darth Sidious has something he doesn't, though. A strength within her, and when they set their wills against each other at the final moment -
(She has actual allies.)
(She isn't, actually, strong in a direct contest. Elesse, for all that she's a baby Jedi, could probably challenge her on an even playing field.)
(By the time Plagueis realizes she's betrayed him, he's already lost.)
Darth Sidious holds herself still in the aftermath of his death, as the dark side of the Force jolts and seizes around her - as that immense wellspring recognizes her as the primary outlet for its fury -
She tries to avoid falling over. That'd probably be bad. At the very least anti-climatic. (She left Maul to watch Anakin. She deputized her handmaidens to manage her persona as the Senator and her work in the shadows as Sidious, put Shmi in charge of demolishing Plagueis's support base with them. Gave to Ridis the job of dismantling Plagueis's primary base and protecting what she can of his archives.)
She has Salacis with her, her apprentice. She has Elesse here, part of the trap she set for Plagueis.
(Perhaps it wouldn't be so dangerous, to let herself be weak.)