According to our commonly accepted (by other people 8)) axioms, Crono and Lucca (and the rest of their timeline) should have been sent to the DBT the moment Marle went back in time. They were not. This is significant. Here we have one of the few glimpses (the only glimpse?) of someone going back in time from the perspective of someone who does not go back in time. According to the TB Theorem, the timeline we the gamer were experiencing should have been relegated to the DBT. Whoops. Guess the theory doesn't hold up all the time. :o
You're talking about the Principle of Discarded Timelines, not Time Bastard. "Kind of" related, but different. Make sure you keep your terms straight.
But you remain correct about this situation. This situation occurred to me yesterday. Only a few explanations:
1. Marle becomes her own great(x10) grandmother. Ugh, but possible due to time traveller immunity. Theoretically this could be an ever-occuring 12-generation traveller. Marle replaces Leene, and has descendants, etc, and so "New Marle" could have had the SAME events occur at the fair with Crono, etc. Crappy explanation, but possible.
2. Marle is discovered as an imposteror, Leene replaces, and Marle lives on quietly not greatly affecting the timeline. Again, a "cop-out" explanation, but possible IMO. Marle only "temporarily" replaces Leene, Frog STILL rescues the real Leene (as in Lavos Timeline), and there's no big problem.
In both cases, the timeline where Crono, Lucca, etc, SEE Marle go back in time gets thrown out, but theoretically the SAME events could occur in the "replaced" timeline, and thus they DO "go after her", though I think there's other problems (even accepting Time Bastard, IMO there's STILL problems).
Now this is NOT the "only" occurance of observing others go back. Same thing happens with Lucca saving her mother, though that one's easier to dismiss as EVERYBODY camping together already had immunity, and thus it is no big deal.
But this requires resolution IMO. What happened? How can this be resolved? ESPECIALLY with the idea of Lucca joining them "partway in". This seems rather fishy as well. New theorum perhaps? Expansion on something else?