It's a separate timeline. The original still exists; it has not been overwritten.
This claim has been made repeatedly, but without any solid proof given.
In all previous time-travel stories in TREK, changing the past has meant the ONE past of the Trek universe gets altered, and has to be FIXED in order to restore the original.
Why would this case be any different?
The only conclusion I can arrive at if the mainstream Trek universe still exists in the 24th century
(and can someone give me proof positive that it still does? where is this said?), is that Nero and Spock were not only hurled back in time, but into an alternate reality.
This would explain a lot, including the Kelvin tech and internal look of the ship.
It was never the "original" Trek universe to begin with. What Nero's changing isn't his own timeline anyway, so NATURALLY the mainstream universe is intact. Nobody's messing with ITS past.
Then again, what if the changes we've seen are, in the overall scheme of things, so minor that it hasn't had any serious effect on 24th century Trek?
Maybe Picard went to see Sarek on NEW Vulcan. Maybe Tuvok's family was ALREADY living on the Vulcanis colony, and didn't die with Vulcan, Tuvok thus being born anyway.
This is why I say "It's not clear", because this could be the past being overwritten, or it could be an alternate reality to begin with, and so far the proof could be taken as meaning it's either.