What about the countless star systems that will be destroyed so long as Starfleet is ignorant of this threat?
But what else changes in the process? That is the thing you can never determine until you've committed to interfering with an event that has already unfolded.
Take your drowning child in the swimming pool. Spur of the moment, I would rush in and save him without a second thought. But five years later? I remember Timmy having a bitching pool where he ended up drowning. So I decide to go back and save him so my younger self can keep using that sweet ass pool.
But, Timmy's parents decided to have another child after Timmy's death. Without Timmy's death that child would have likely never existed. So I saved one person but I sentenced another to non-existence.