Answer to the last question of your is yes, in theory is possible. But everyone is different. I am a regular Morphine user (four years physicaly addicted, two years on M.), and I have job, wife and kid and hobbies. The thing is, I never used opiates or opioids to enjoy 'flash', and to spend day laying on the couch. I mainly use them to be functional, so stop pain and discomfort, physical as psychological one.
I am not sure what are methadon alongs you refer to, but methadon is maybe even officially the worse opioid used for the maintenance therapy. I have slides here, used for education of general practitioners, which show price as the only pro of methadon. There are many cons.
Considering brain/cognitive problems, I am not aware of a study which showed that long term use of opiates will cause brain damage or cognitive issues. Even if there is such study there are also studies which show (what is also my personal observation), that there are no signs of any cognitive or intellectual issues by longtime Morphine users. The first book (It is from the country I am from.) where I found such an observation was called 'Medical Psychiatry' printed in '70 (seventies).
At higher doses one is definitely a bit impaired, but you would wonder how sane a totally knocked out person (during flash for example) can sound or be. Try to communicate with someone in such state and you will see. If he can hear you, and is able to speak, it will give you pretty sane answers. Nothing similar to alcohol intoxication. I would compare this state with a sleep or dreaming. I mean it has a similar influence on our state of mind in regard to our mental capabilities. When you wake up someone, depending on the dreaming phase he was in, the person will need a bit time to collect one self, and will be able to communicate and think quite well, but definitely not at 100% of his/her capacities.
Unfortunately the thing is not that harmless, as it maybe sounds when one reads what I have just written. There are issues, but mostly manageable, and they don't have to leave a permanent impairment... Like osteoporosis (usually termporary condition), digestion is slowed down (This can cause serious issues if not treated well.), there is also some influence on cardiovascular system, but usually also only a temporary condition.
Addicts can have quite normal lives if they want to, but I think these are the same people who would stopped being addicts if they would be forced into that. People who only care about drugs and to get high are simply put selfish assholes, and there is no cure for that.
