Delphine is a victim of her upbringing and the social norms of the time, the very same way first generation Sanson's wife, Marguerite was. With Olivier having handed her the key to the unlock the belt before he went to jail, there's no certainty or assurance that her future would have looked any brighter had she "stepped" away from the given role of the old master's widow and followed after Olivier.
Despite the mass majority supporting Olivier, she's afraid of what comes after, because nobody foresaw the execution being stopped. The reason Delphine had "chosen" not to take off the belt is the most extreme answer she could have given to him, that she would remain in the house, now with more agency than she has ever had in her life. She's now the Martha of the household. Even if she had regrets it was already too late to back away.