since all brain receptor become less sensitive as you expose them to their target chemical, doesn't it means that drugs will always cause some damage and be addictive ?
Addiction and habituation are not just down to this effect either. I know a variety of people who have been both daily pot smokers and daily tobacco users. The tobacco use was far harder to stop. (I'm not going to pretend pot doesn't have some addiction potential, and in fact it does seem to have minor withdrawals, just not anywhere near the level of tobacco, opiates etc). Receptors also recover after varying amounts of time.
So I don't see why we couldn't research things that are fun, don't compel people to chronic use, have minimal impact with occasional use, etc etc. It may not be possible to make the 'perfect' drug, but it may be possible to eliminate many of the harms. Particularly where some drugs are toxic over and above their primary effect, have toxic metabolic products etc
However, there are many ways of affecting receptors further up the metabolic chain. For example, recent research has focused on neurohormones are alternatives to benzodiazepines for anxiety treatment.
Some of these may be, in theory, less addictive or entirely non-addictive. Unfortunately, the current drugs are very effective for short duration courses, the most common medical use.
A less restricted market would incentive drug companies to find less harmful substances for recreational use. In a regulated market, you could even use the tax burden to further shape the public's preferences.