It appears that her election as Chancellor depends on her party winning the control of the parliament (either by majority of members or leading a coalition), presupposing that she is elected in her district.
The voting system is complex. People vote for a local representative and for a regional party list. The parliament is filled by a combination of these. Some candidates are on both lists in case they lose their local seats (then they go in as part of the regional list in some cases).
If Merkel becomes unpopular nationally then the voters will have to reject her party at both local and regional levels to the extent necessary for some other party to win power and then that party’s leader will be Chancellor.
Maybe her slogan will be Deutschland Import Alles, or Ein Reich, Swei Volk, Ein Furore.