I'm sure this has already been discussed, but I couldn't find anything.
Old Janeway's selfish time travel, just because she had been too stupid... would the Temporal Integrity Commission introduced in the show really just sit back and let that happen? What she did fucks up the development of the entire galaxy for 25 years. All the first contacts Voyager would have made are now not getting made, and the Borg are simply wiped out.
If not the guys from the 29th century, surely the Temporal Investigators from DS9 would kick her ass, and certainly wouldn't let her get promoted to Admiral, wouldn't they?
Unless everything Admiral Janeway did in "Endgame" was a better outcome than what they had.
For example: what if going back and saving Seven, Chakotay & Tuvok brings more advantages to the Federation/Starfleet than the future we saw? Bringing home the EMH sooner would allow Starfleet medical to advance their knowledge and technology years sooner.
But that is a pretty egoistic view (just as egoistic as Future Janeway's actually). If there's indeed a time police, they would, or should, screw them, no matter what. They violated the timeline. Future Janeway destroyed the lifes of billions of people when she went back in time. That's a crime. Who is she to change the life of billions just because of a few friends?
What about the Delta Quadrant? She deprived the species in the Delta Quadrant from making contact with Voyager. She changed the fate of the
entire galaxy. And in Star Trek, that is not her decision. That is no one's decision to make. I would consider that a crime. And that's why there is such a ship as the Relativity in the Trek universe.
So why didn't she go back in time to prevent Voyager getting lost in the Delta Quadrant in the first place?
Why didn't she go back in time to prevent the Dominion War?
Questions like these are the reason why time travel from the future to change the past is always wrong and should never be allowed.
Kirk's time travel in The Voyage Home is something entirely different. He did not change the past. If you believe the scriptwriters, then Kirk returned to the same universe he came from. He simply fullfilled what had already happened, the took two whales and one human from the past and brought them into his own present. He didn't bring anything from the future into the past, like Janeway did.
It would have been wrong if, 25 years later, Kirk had decided to go back in time and destroy the Probe in Earth orbit. Because that's the future changing the past, and he would have taken away 25 years of billions of lives everywhere in the galaxy.
It would have been wrong had he prevented the extinction of whales back in 1986, because then he would have changed the lives of billions on Earth, and subsequently for the whole galaxy for 300 years.
Maybe the Future-Janeway timeline WAS the Prime Timeline, and the one in which Janeway gets home early is a new altered timeline? That would mean that Nemesis is also in an alternate timeline, explaining several things about that film.
I like this theory!
That's also my opinion.