Back In Timeline 1

What becomes of timeline 1 if a time traveler departs that timeline and enters timeline 2? Imagine two horizontal parallel straight lines, each extending from point A to point D with B and C as intermediate points:

the lower line represents timeline 1;
the higher line represents timeline 2;
C represents the moment when a time traveler departs timeline 1;
B represents the moment when he enters timeline 2.

Clearly there is no reason why timeline 1 should terminate at C. Indeed, we have already stipulated that timeline 1, like timeline 2, extends as far as D. The horizontal dimension represents a first temporal dimension whereas the vertical represents a second temporal dimension:

in the first temporal dimension, timeline 1 remains in existence until D;
in the second temporal dimension, the entire timeline 1, from A to D, ceases to exist when the entire timeline 2, from A to D, begins to exist;
inhabitants of timeline 2, familiar only with the events and history of their timeline, would say, if asked, that timeline 1 did not exist and had never existed;
anyone who did not understand that two temporal dimensions were involved might alternate between saying that timeline 1 has ceased to exist and that it had never existed.

The blurb on the back cover of SM Stirling's Dies The Fire (New York, 2005) informs us that the Change occurred after an electrical storm above Nantucket. After reading the Nantucket Trilogy, we are back in timeline 1.