While people may not be willing to kill or rob someone to get a pack of smokes, there will absolutely be a black market for them that will be rife with unsavory characters that will. I live an hour away from a recreational marijuana state and it has destroyed the local black market for pot as anyone who wants to get high can just drive across the state line and get their own pot. No more sketchy drug dealers pushing other substances, no more police stings to catch teenagers buying dope.

Buying controlled and reasonably dangerous substances from licensed retailers like a dispensary, grocery store, or even a gas station is a lot safer for everyone than trying to keep tabs on a black market. The danger of prohibition wasn't alcoholics trying to find their next drink, it was mobsters like Capone trying to dominate the black market for popular goods.

Edit since i misread what you meant: Sure tobacco is on it's way out as is, but nicotine consumption is still skyrocketing. I dont see how banning tobacco sales for anyine born after a certain date like Sunak is proposing will help anything. People under 30 are already way less likely to smoke tobacco but consume unhealthy quantities of nicotine anyway.