The Traveler isn't a good Machine. It stands for life, propagation, bounty, and rebirth, but it has been made abundantly clear that the philosophy it conducts itself on dictates self preservation above all else, as to be able to continue it's eternal journey throughout existance. As such, if presented with overwhelming odds of defeat, it will abandon the system and species it happens to be consorted with at the time. I hypothesize the reason the Traveler didn't abandon Sol was due to a surprise attack implemented by the Hive while in collaboration with the Darkness. The Traveler might abandon it's work when all is lost, but not a moment before. When enemy forces arrived in the system, the Traveler immediately suspended it's work on Io, retreating to Earth, where it's odds of survival were the highest. I believe these first attacks were slight of hand, to keep the Traveler and Humanity's attention while the Darkness encompassed the system. Once all the pieces were in position, they sprung their trap. The Traveler couldn't flee as it did so many times before, and was left with no other choice but to cripple itself to deal with the immediate threat of being consumed by the Darkness. The Ghosts are preventative measures the Traveler released so that it would not be left unguarded while it healed, not a blessing onto Humanity, as so many see it as. If the Guardian's campaign against the Darkness is successful, I see no reason that the Traveler would leave. With no threat of destruction, the Traveler would be free to resume it's work in Sol, but if conditions in the system continue to worsen and the Traveler garners enough strength to leave on its own, I would not count on it's presence in the comming battles.