The problem with 'the Kelvin changed Starfleet's mind' is that plenty of ships are lost to powerful foes, and nothing much changes. After the Doomsday Machine attack, Starfleet didn't redesign the fleet. After the Defiant was lost to Tholian technology, Starfleet didn't change gears. Starfleet loses ships and crews by the handful, and only in the face of wide-ranging threats do they ever significantly try to adapt their ships (ie. the Defiant, to face the Borg).
It just makes more sense to say that Spock and Nero went backwards and sideways, rather than straight back.
It just makes more sense to say that Spock and Nero went backwards and sideways, rather than straight back.