Airlines make money by standardising. Any airframe used for smoking flights would immediately become non standard. Certainly, if I were to board an a/c that stank of smoke and was told that this was a 'smoking permitted a/c' that was now operating a non-smoking flight, they would have to find my bag in the hold as I would refuse to travel. So, to make my life simpler, if I found that a carrier was having some smoking permitted flights - I would never book with them.
The restrictions on long haul smoking have been around for 15? 20? years. When they started making them non-smoking, things like nicotine patches and chewing gum were not readily available and now they are.