they were experiencing it as though it were a real world with the ONE BIG EXCEPTION.
there was no evil let loose upon the world with the destruction of the Island.
If we assume that the sacrifice of Locke and Jack and all others who died protecting the Island in the real world means anything, then the Alt timeline cannot be even a rough approximation of the real world with a sunken island as there is no supernatural evil running rampant within this timeline.
That, alone, makes the alt timeline almost totally worthless as anything other than a way for ghosts to come to grips with their lives.
And, full disclosure, I admit I kept waiting to see what would be the dark side of this seemingly happy timeline. Damon hinted at this in an interview; that the destruction of the island was a selfish act and would have ramifications beyond its affect on the 815 passengers
but nothing. it was just a place for these same myopic people to find themselves before moving on.
thats my biggest disappointment. I thought there was a lot of potential for this alt timeline to be a real place with a smokey on the loose (even if its not immediately apparent) and they never did anything with that, despite that swooping underwater shot in LA X.