It really depends both on what you mean by "lower tier" drug, and what people are seeking, and how frequent users they are. But I suspect a reliable potency al

It really depends both on what you mean by "lower tier" drug, and what people are seeking, and how frequent users they are. But I suspect a reliable potency alone would do a lot for slowing the upward creep than shunting people to a drug dealer who might decide you'll be a more profitable customer if they raise your expectations of the high to seek by selling you "heroin" without telling you it's not pure heroin but cut with fentanyl (tens of times as strong) or carfentanil (thousands of times the potency). Even with an upwards creep, there are vastly different strength levels and harm profiles for different opioids, and a strategy of what is available focused on slowing advancement and steering around the most dangerous options might well allow for keeping people on far milder options for far longer and/or give more options for shunting people onto interventions if they want to stop. The first step is really to accept it's a healthcare problem, and recognise that it's a worse healthcare problem if people go to drug dealers because you refuse them. If so, the question becomes not necessarily about a free for all, but about how many hoops you can get people to jump through to shunt as many as possible in a healthier direction without making too many decide it's easier to buy from a dealer.