It is a matter of record that had the two towers not come down on Sept 11th 2001 they would have had to have been closed shortly thereafter anyway as they where riddled with massive ( millions of tons) amounts of asbestos used in their original construction, the use of which was about to become completely illegal in building used as places of employment in the federal United States.
The cost of replacing the the asbestos in the towers was estimated to run in to tens of billions of dollars, and like wise the cost of demolishing the towers safely with that much toxic material in them would have been equally expensive.
The plan therefore was to simply abandon them and leave than as concrete sealed monuments, from 2005 onward.
When the towers did come down clouds of asbestos flooded the surrounding areas causing massive outbreaks of asbestoses. A second toxic dust cloud was sent up weeks later during the clean up operation, and again intermittently over the following decade, as ground zero was cleared.
This is not conspiracy theory, there is no assignment of blame or accusations of cover ups, these are simple verifiable (but largely ignored) facts.