Just watched "Endgame" for the first time.

A brief rundown of standard time travel. Basic assumption is if one can travel in time, all time must therefore exist; and like a coast to coast highway, we perceive it as linear (past to present to future, through a series of causes and effects).
The basis of the federation rules is fear of the "Butterfly Effect", where going back and affecting past events can have major, possible negative, ramifications on the present. Whether one can or should travel, and the effects it might have, is the foundation of most stories concerning trips through time; it's at that point when rules may vary (the effect meeting one's younger self may have; how much the past can be altered; alternate/mutiple timelines; can the past be changed at all).

The fundamental problem with altering the past is the creation of paradoxes; if one travels to the past to make a specific change and succeeds, once returned to the present, the reason for travelling in the first place has been negated, no need to go back and make the change; but if you don't go back, the original problem remains (the need to go back, then no need to go back, then need to go back ad infinitum).

Paradoxes are the big issue that most writers have to deal with; how good the story, is often determined by how well the paradox problem is handled. Voyager was always sloppy in this regard.

In 'Endgame' Adm. Janeway goes back to change events, and succeeds; consequently, she should have ceased to exist. Some would theorize the she didn't change her own past, but created an alternate timeline; my problem with this completely viable theory is that the viewer is now asked to follow a different timeline from the original, solely to let the writers off the hook. In any case, the producers of the show just decided to ignore the issue altogether(they often did this); sloppy, lazy, and somewhat insulting.