Katrina and the mass forced migraton of NOLA Amish to inner and south Houston, did ensure the open-air drug market had a permanent supply chain. There were a lot, like a lot a lot, of hoodoos that refused to expand outside of NOLA until Katrina forced their hand.
Now though, los pendejos have have taken over South Houston, Pasadena ( Amish paradise! ), around Hobby, where are the Amish gonna go? Spring? The Woodlands?! CONROE! LOFL.
Back then, the drug gangs in New Orleans were not affiliated with the west coast "super gangs". Instead, they were entirely identified with public housing "turf", the various housing projects around the city. The crews from the Desire Projects, Fischer Projects, Magnolia, B.W. Cooper, Melpomene or wherever... they were each formidable enough that the Bloods and Crips didn't even attempt to show their colors in New Orleans. Only the Latin Kings had a foothold in Kenner, Louisiana - a western suburb out by the New Orleans airport (and then the largest hispanic population in the state, though in 2005 that was actually declining).
So yes, it is true that the New Orleans drug gangs didn't want to operate outside of the city. It was bad enough when renovation of one or another of the housing projects was planned - the city would move residents from one project into vacant apartments in another project. Result: instant bloody gang clashes. Crabs in a bucket.
I'm not sure what the gang picture in New Orleans is these days, though I suspect that some of the multi-state groups are now active there. I'm also not sure how the big picture of Houston crime can still be neatly blamed on Katrina. Pre-2005, Houston did already have a thriving, even vibrant ghetto thug culture, complete with lots of drug-related shootings and other fun activities for those dazzling urbanites. The die was already cast.