You can see even more reasons above why I don't like timetravel episodes.
I hear you. I share your sentiment about time travel stories, although I have to say that I am glad that I dug back in time to find this interesting thread.
But, what's so precious about the timeline that it couldn't be messed around with.
I agree with you, for the most part, on this point as well. Unless someone is affected the way Kirk and the landing party were in "The City on the Edge of Forever" by being marooned in oblivion, why should a particular time line be considered sacrosanct?
Why should one time line be considered legitimate and another not so?
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What really bother me about "Yesterday's Enterprise" was Tasha's behavior. She asked Guinan what happened to her in the other time line. Guinan told Tasha that Tasha was not suppose to be "here". Essentially Guinan told Tasha that Tasha was illegitimate to be alive. At that point, imo, Tasha should have told Guinan to "F**k off". Instead, Tasha accepted everything Guinan told her and believed that she was undeserving of living any more.
If someone told you that you really shouldn't exist because someone messed with the time continuum, you might ignore them or tell them to get lost. Even if what that person told you may be true about someone messing with the time line, so what?
Tasha's behavior was not credible. Tasha's reality is the one in which she lived in. Why should it matter to her about her other self in the other time line? Why should she care what the woman in the stupid looking hat said? I don't believe people would behave the way Tasha did. It didn't make sense that Tasha would sacrifice herself to fulfill some abstract theory about time lines.
And what if in a different scenario, say, the Enterprise C by travelling into the future had prevented the Dominion war from ever happening and preventing all that destruction from happening, then would that be a good thing? Would it be moral for the Ent C to go back to where they originally came from and let the war happen?