It has been postulated that the poor condition of the building that Christian meets Sun and Frank in is evidence of an altered timeline in which the Others never moved into the Barracks.
This idea assumes the following that the building that Christian shows Frank and Sun the photo in is in the Barracks, thus indicating that the Barracks were never inhabited by the Others, demonstrating a change to the timeline.
I believe this assumption is in error, based on the fact that Pierre Chang asked Jack who his Shuttle Driver was. I realise the Recruits are a little groggy from the drugs they took to knock them out for the trip to the Island, but doesn’t it strike you as a little lazy to have a shuttle service from the dock to the Reception building if the distance between the two was the 15 feet that Frank and Sun walked?
The obvious explanation is that the building in which Frank and Sun were shown the photo is not the same as the building where Jack met Pierre Chang.
There is enough circumstantial evidence rather, to assume the following:
The submarine brings new Recruits to a dock. Recruits get off the dock and mosey over to a Reception area, the one seen in TMBTC when young Ben first arrives on the Island. This Reception area has a building with group photos of new Recruits, including the photo from 1977 that Christian shows Frank and Sun. The Recruits are then shuttled to the Barracks, which are some distance inland from the Submarine dock and not on the shore. Upon arriving at the Barracks, the new Recruits go to the building where Jack meets Candle, and they are given their work assignments and change into their uniforms and after taking a picture are treated to hamburgers and punch.
After the purge, the Hostiles move into the Barracks and remove all the DHARMA paraphenellia, but they leave the Reception area to fall into disrepair and it assumes the same old, nasty worn out appearence as the rest of the DHARMA stations that we have seen, such as the Hydra, which retained it’s DHARMA decorations even though the Others used it fairly often.
When Sun and Frank arrive at the dock and meet Christian, this is at the unused Reception area, not the Barracks, which is a shuttle ride away. The building that Christian brings Frank and Sun to is the unsued Reception building that the Others let slide into disrepair.
Before I finish, let me anticipate the possible counter-argument that the building where Jack meets Chang has photos on the wall, as does the building where Christian shows Frank and Sun the 1977 photo. My rebuttal:
I have photos hanging in my house, does that make my house and the building where Frank and Sun see the phot the same building? No.
There is therefore no evidence of an altered timeline or any other less mundane explanation than presented here.