Am I the only one who doesn't assume the destruction of Romulus is an automatic given in the main continuity we've been following all these years?
A number of people see it the same way you do, but it doesn't track with what we know. The intent of the filmmakers was that Spock Prime came from the same reality we've been following from the start -- that Romulus was destroyed in that reality, causing Nero and Spock Prime to go back in time and split off a new, altered reality in parallel with it. The whole reason they included the time-travel aspects and coaxed Leonard Nimoy out of retirement was to make the new timeline a direct continuation/offshoot of the old rather than something completely unconnected to it. If that hadn't been their intention, they wouldn't have bothered with all that messy time travel at all. The Spock that Nimoy played in 2009 is the same Spock he's been playing since 1964. He's just living in a different timeline now. But that only happened because Romulus was destroyed in his original timeline.