Consider the “Iron Law”
The "iron law" doesn't mean that without prohibition, the stronger versions wouldn't exist. The law implies that under prohibition, the stronger versions will dominate the market, whereas without prohibition a greater range of options will be available, and many consumers will choose the weaker versions because that's what they prefer.
This is very clear with alcohol. There have always been high-strength spirits, but in most countries where alcohol is legally available, wine and beer are more widely drunk than spirits. Many people find them more suitable for socializing, drinking with meals, etc. In Saudi Arabia, beer is hard to find because it's difficult to smuggle in the required quantities. Smugglers bring in whiskey and vodka because, in terms of alcohol content, 1 bottle = a whole case of beer, or about 6 bottles of wine. So that's all that's available to most Saudi drinkers.
With opiates, before prohibition, opium, morphine and heroin were all on sale in the pharmacy. Opium was by far the most popular with recreational users, because most people prefer something milder that isn't just going to put them straight to sleep. After prohibition, heroin quickly came to dominate the illicit markets because in terms of potency, 1g of heroin = 3g of morphine = 30g of opium. So today, most opiate users in the UK only ever use heroin, because nothing else is available (it's a bit different in the US because morphine and oxycodone are prescribed much more freely there, and prescribed drugs leak onto the illicit market). In the last few years, fentanyl has become very popular with drug dealers, because 1g fentanyl = 50g heroin. It's increasingly dominating the marketplace because drug dealers find it easier to smuggle, even though most users prefer heroin. Even more alarmingly, some dealers are using carfentanil, 1g of which = approx. 3000kg of heroin. The more the fentanyls dominate the marketplace, the more users die because they're very difficult to dose accurately.
Perhaps the best way to answer your question is to look at the process in reverse: what happens when a prohibited drug is legalized? During alcohol prohibition in the USA, the market was much like Saudi today: lots of whiskey but not much else. After prohibition, the market looked much like that in any other western country: wine and beer are the most popular drinks. Now that cannabis is being legalized in an increasing number of states, you can see the same phenomenon. Legal cannabis dispensaries sell a whole range of types, and many consumers choose weaker variants because they don't want to get so stoned they can't stand up. There is even a lot of demand for cannabis that's barely psychoactive at all, but which helps with various medical problems. These are the high CBD strains with a THC/CBD ratio of around 1/10 (the higher this ratio, the stronger the cannabis - skunk has a ratio of around 20/1 or something like that). Before legalization, there was much less choice and stronger strains dominated.