What happens with the old (pre-reboot) adversaries largely depends on what they can pry out of Spock Prime.
If he shares the secret of the red matter, for example, or Genesis, Starfleet has the power to not really worry about the Borg anymore, or V'Ger, or the Probe. Or, really, the Dominion. Or the Doomsday Machine.
Why should Startfleet have to "pry" anything out of Spock? He isn't in his timeline. "Temporal Law" rules (whatever they frak they may be) do not apply.
Spock should give Starfleet the full download. That doesn't guarantee anything, of course. The Botany Bay and the Doomsday Machine might be right where he says they should be, on such and such a date, extrapolating from their trajectory which of course Spock remembers right down to the last decimal place. Then Starfleet blows them straight to hell, problem solved.
Or the changed timeline might have already affected their destinies and an entirely different and unpredictable story transpires. What if Khan has already allied his merry band of superhuman muties with the Dominion? What if Nero stepped on a butterfly which set off a chain of events that had the Borg taking command of the Doomsday Machine? The fun never needs to end!
Quark's grandpappy and Grilka's grandmama made an ahem alliance...wow, did that change things...and you thought the Borged Doomsday Machine was scary...The Klingons and Ferengi working together?