The time line that leads to the Temporal Prime Directive, of course. Once you have the power, you want to make sure nobody can take it away from you by changing the past that led to your having the power.
I don't trust anyone with that sort of power. Not even the improved humans of future Starfleet.
Besides, look at some of the temporal changes in "Year of Hell." Suppose you could change history so that it remained largely the same, but the Dominion War never happened? There's still a peace between the major powers (to some degree at least), but the thousands of casualties never happen. Why should this alternative be necessarily worse than the normal timeline?



