So why didn't Spock Prime (in TOS) also take a starship around a sun and restore the planets destroyed by "The Doomsday Machine"? The Starfleet starships destroyed by "The Ultimate Computer"?
Are you asking this seriously? I'd have thought the answer here was self-evident.
To wit: those disasters were
not the result of time manipulations in the first place. The ethos we've seen consistently throughout the history of Trek is that time travel intended specifically to
alter the past is bad (even for desirable ends), but that time travel intended to
correct or
minimize alterations of the past is a worthwhile endeavor.
It's a matter of being sensitive to context. By way of analogy, we all understand why it's considered wrong to initiate violence against someone, but it's permissible to engage in violence
if it's a
defense against a threat to self or others.
(Philosophically speaking, the underlying view would seem to be that at any given moment in time, the future remains to be shaped, but the past is a known quantity that should be left alone. Presumably the idea is that the unintended consequences of trying to change time can dwarf any positive change one might hypothetically achieve.)
I can only think of two clear counter-examples, or perhaps I should say "violations," of this ethos. One was
STIV:TVH; the trip back to retrieve the whales was technically intended only to bring something forward that could preserve the future, and the stakes were certainly high, but it's undeniable that while in 1986 the Enterprise crew didn't take the kind of pains they should've to avoid creating paradoxes and/or unpredictable side-effects. The other was
the finale of Voyager, which was IMHO a far more serious breach of "protocol" inasmuch as Janeway's actions knowingly and deliberately wiped out sixteen years of Federation history just to get her ship and crew home sooner, without any apparent concern for what the indirect consequences might be.
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(BTW, just as an aside... those here who keep insisting that the Many Worlds Interpretation is the only "scientific" approach to understanding time travel, and who are anxious to reinterpret and retcon the entire history of Trek in light of same, really ought to take a step back and read up on the Novikov Self-Consistency Principle. Google is your friend.)