You don't give the engineers much credit do you? The design specifications included the structure should withstand the impast of a B-707. The reason for having a sprinkler system was to protect the structure from a catatrophic event, such as the impact of a jet aircraft. Redundancy in design would have negated the impact of the aircraft, in my opinion.
Please consider that three years of planning was but in the mission to bring down the towers. If that is the plan and the mastermind of the attack is an engineer, is it a stretch to consider that the plan included sabotage of the sprinkler systems that helped minimize the destruction of the 1993 attack by the same people? These folks learned from their first failure.
Just going by what I've seen the engineers say regarding the attack.
impast of a B-707
Which didn't weigh as much as the fully loaded aircraft that hit the buildings...according to the engineers.
The reason for having a sprinkler system was to protect the structure from a catatrophic event, such as the impact of a jet aircraft
Find me a sprinkler system that can deal with the amount of Jet-A which was suddenly introduced into the structure via airliners travelling at high speed.