she explains in the episode that she targets ships that were going to be destroyed if she didn't time travel. it was their way of avoiding altering the history -- only taking stuff that would be wrecked anyway. they were going to hit the dark matter storm anyway.
besides, even if things had been the way you said, you would still have an impossible paradox... if she never showed up, then they would have never known the time portal existed, and therefore would not have destroyed it, which means she would have still been able to travel to the past and cause the whole thing to happen.
Agreed that it was a paradox and something I pointed out in my original response. This is also where writers get into the liberties of alternate timelines since we know time travel isn't a real science yet.
We know the Orville was destined to be destroyed when it entered the dark matter field. We know Pria enters the wormhole sometime around 2900 to go back into the past to save the ship. When she does this, she sets into motion a chain of events that leads to the Orville surviving and as a result, the wormhole is destroyed.
At this point we are dealing with at least 2 timelines, the one where the Orville is destroyed and one where the Orville destroys the wormhole.
Once the wormhole is destroyed, the convergence of these two timelines becomes impossible. This means Pria can no longer exist in the past because there is no wormhole in the future for her to enter (of this timeline). Thus, she disappears.
The paradox comes into play because the Orville can't just disappear right, because if it does, then the wormhole remains open and we're back to it eventually becoming destroyed. This means we're left with outcomes that you could consider logical.
1. Everything resets, Pria never arrives in the past, and the Orville is eventually destroyed. This basically hits a recurring infinite loophole where the Orville destroys wormhole, Orville is destroyed, Orville is saved, etc.
2. The outcome we got which was probably far easier for the writers to create and the viewers to digest on a show like this is the ship was saved because of Pria never arriving in the past because she can't in the future she created.
My point still stands that it really is a great show and I thought they handled the episode with a bit of finesse when it came to time travel. That being said, you stated it was really bad and I would certainly be open to hear what would have been a better ending or explanation.