Whats there really for mexican and black gangs to take over. their not into gambling and they dont have any ties to any unions and labour activity. Your local street level dealers would probably have to go to them for their drugs, worst case the gangs start taxing local drug dealers and criminals, but i doubt they are gonna set up shop in some white neighbourhood where they stand out. The local officials and politicians would probably go the extra mile to make sure that wouldnt happen more than they did when it was getting bad in the hoods the gangs originated in. I doubt you'll see any drastic change if the mob was completely gone, prob low level gang activity but i dont think youd get the same level as you would in a place like harlem or compton.