Sentience is a computer. Despite the fact it could think, rationalize, and had the power to keep us all here, the weaknesses of it are inherent in the fact that it was a machine built by ones it had imprisoned.
[Thinking]
Keeping the Engineers here probably made it even more vulnerable since they know the codes and protocols to hack and alter its system. They replaced Sentience with Acumen, and we don't even know why. Perhaps they realized Sentience was beginning to sympathize too much with the residents and sought to circumvent that.
[And now he's off in his own world, thinking and trying to make sense of all of the strange things going on lately.]