The whole concept of Denizens is strange if you think about it. On the other hand, we’re supposed to disagree with the Apex for thinking of them as things to be used...but on the other, the Train literally exists just to help/challenge Passengers. Sure, plenty of Denizens are living their own lives, but how many of them realize that their worlds only exist as window dressing for the humans’ adventures?
But not all of them do. Marcel was a villain for trying to keep Jesse and MT in his car forever, but think about it: once they leave, his car presumably resets for the next Passenger, so he has nothing to do until someone else comes. Can he just leave? Terrence escaped from the Toad Car, so good for him, but is it just empty now? Morgan can’t go anywhere, and she’s clearly unhappy with her role. Et cetera.