Had the towers not collapsed I think that the damage would possibly have been repairable.
The damage from the 1993 bombing looked extensive at the time but they were able to get that most of that fixed reportedly within just a few weeks
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Following the 1993 bombing damage to the below grade levels of the towers and the Marriott (then Vista) hotel was fully patched up within two years.
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The towers looked mostly intact below the impact zone prior to the collapse, even if there were some localized floor failures taking place that were obviously hidden behind the exterior cladding. For example, lighting was on in the towers even after the south tower was hit and many people inside allegedly had no indication that the towers would collapse even after the initial explosions from the plane impacts and jetfuel blasts down the elevators.
The toughest part would be extinguishing the fires, although many of the firefighters were getting close to at least the mid portion of the towers right before the south tower collapsed and the remaining firefighters in the north tower were given the command to exit the building.
One thing that is confusing is that despite there being fire extinguishers on every floor of the stairwells and helicopters that could potentially hose down fires, none of these methods seemed to be even be remotely successful in clearing out even small pockets of fire. Perhaps the fires were too extensive in floor coverage and spanning multiple floors that it required full scale deployment of firefighters to the scene to even begin to extinguish the fires in sizable enough areas to allow escape.
On a more somber note, had the towers not collapsed it would have probably been far easier to identify the remains of jumpers and the passengers aboard the hijacked airplanes. The plaza would still be intact, as would the underground mall. The lobbies could be repaired.