Earth/Krypton's destruction really didn't matter.
Humanity had colonized all nine planets (at the time of the graphic novel's writing, Pluto was a planet) thousands of years previously. It's likely that we had left our solar system long after, and we'd already gone into the afterlife (why someone didn't start having Luthor help is beyond this troper). So yes, humanity's homeworld was gone, but in its place is a human empire.- All the same, it's a pretty big loss of life.
- Loss of the galaxy's governments, check. Destruction of the birthplace of humanity, check. And the loss of the worlds most brilliant minds, and I'm pretty sure the earth was the elite of the worlds holding on to the best and brightest minds, and it's all lost.
- It matters morally and metaphorically - a whole planet self-destructing because utopia is fools' gold.
The Firefly 'Verse is in this universe.
Luthor decided to make a Plan B, building tons of spaceships and sending them out to colonize another solar system far away from Earth. Over time, the story morphed into one of Earth's resources being used up and them leaving. Luthor had an affair with a psychic, creating the Tam line. The baby that was sent back in time isn't going to become Superman, he's the Russian Batman.
Gotham is part of the U.S.A. so why would Batman's origin happen in Russia? As an extra theory, Gotham's Bruce Wayne never became Batman here because of the events happening in America at the time. Phantom Zone
Jordan Luth became the first person to travel to the afterlife. Over time this achievement was used to banish criminals, and the dimension travelled to became known as the Phantom Zone. Brainiac didn't really have a self-destruct mechanism.
Luthor jury-rigged one while he was inside Brainiac. A self-destruct mechanism, let alone one conveniently designed with a public countdown so Superman can (and must) carry it off into space is counterproductive to Brainiac's programmed objectives, as it risks destroying the information he has accumulated.