[[WMG: Jean Luc Picard not wanting to destroy the Enterprise is more than just revenge.]]
He was keeping a promise he made to [[Film/StarTrekGenerations Kirk]], not to let anything getting him off the ship.
[[WMG: The Crew of the Enterprise is the source of divergence into the Mirror Universe.]]
The ''Enterprise'' crew was present to ensure that the events of First Contact went off without a hitch. Along the way, they managed to inspire Zefram Cochrane with tales of the future and how humanity manages to get past the post-apocalyptic hell-hole they are currently in. Thus informed, he greets the Vulcans in peace. Now, imagine how Zefram would have been with no Borg attack and no ''Enterprise'' crew there: a drunk depressed asshole living in the remains of what was once a superpower nation's nuclear arsenal. When aliens descend from the sky, using a highly advanced starship with who-knows what kinds of weapons to use on the poor, insignificant humans, odds are that he would react in a more wary, more paranoid, perhaps more violent manner (perhaps by shooting said aliens with a sawed-off shotgun and looting their ship) due to fears over what will happen now that highly advanced aliens have landed in the aftermath of World War 3. Thus, the crew SetRightWhatOnceWentWrong and ensured that the main timeline, rather than the mirror universe, became the 'prime' reality.
* This is presented exactly that way in the beginning of ''[[Series/StarTrekEnterprise Enterprise's]]'' "In a Mirror, Darkly" two-part episode: the Vulcan ship lands, the Vulcan greets Cochrane, and then a refilmed version of the scene shows Cochrane (or his hand double) drawing a gun and shooting the Vulcan before the rest of the crowd rushes forward to loot the ship.
** Actually, this theory is '''Jossed''' by "In A Mirror Darkly". Mirror-Phlox clearly states that he's looked at the historical records of the USS Defiant NCC-1764 and found that [[NeverWasThisUniverse there never was any specific point of divergence between the two universes throughout recorded history]]. The timelines moved in parallel but were never the same, with the Mirror Universe's history always being distinctly more brutal than the "main" universe (with the exception of the works of Creator/WilliamShakespeare). The opening sequence with Mirror-Cochrane simply illustrates the difference between the two universes.
[[WMG: The ''Phoenix'' test flight would have failed were it not for the intervention of the ''Enterprise'' crew]]
To be painfully blunt, Cochrane was a drunk and mentally unstable. The ''Phoenix'' was cobbled together from salvaged parts and a converted nuclear missile. In the ENT episode "Dear Doctor", Archer is adamant that the technologically advanced, peaceful Valakians just don't have the necessary skills to handle warp drive technology. Yet ''humanity'' developed it in a post-WorldWarIII shanty town built around an abandoned missile silo!
Enter the crew of the ''Enterprise-E'', who are determined to ensure that not only does the ''Phoenix'' test flight go off without a hitch, but they also want to make sure that it happens at the exact date and time required to cause first contact with the Vulcans. To this end a bunch of late-24th Century starship engineers swarm over the ''Phoenix'' and make sure that it is in perfect working order in time for the historical flight. But this could be seen as a case of a StableTimeLoop and YouAlreadyChangedThePast.
The ''Phoenix'' does work perfectly, as one would expect given the fact that it received extensive pre-launch service from engineers from 300 years in the future! One is left to wonder if, had the Borg not led the ''Enterprise'' on a chase into the past and done enough damage to the launch site to justify sending in engineering teams to work on the ''Phoenix'', whether the actual vessel, as constructed by Cochrane and his followers, would have actually ''worked'', or if it would have succumbed to some mechanical failure. Lily Sloan seemed to be the only person in the whole settlement, including Cochrane himself, who took the ship and the flight seriously. Cochrane appeared to be running on drunken ambivalence and constant prodding from Lily. The actual test flight appeared to have nothing even resembling the usual rigorous procedures of a space launch.
If the whole TimeTravel incident had not occurred, it seems as if odds were good that the ''Phoenix'' would not have worked, Cochrane and Sloan would have died during the failed test flight, and first contact with the Vulcans would never have happened.