Earlier quoted context omitted.
Why do you think marijuana legalization will substantially change anything in your park then?
My impression is that weed is their main source of revenue. So legalizing it would make dealing A LOT less profitable. Also, the group of potential weed buyers is pretty large and diverse, meaning they need to approach pretty much everyone. The group of potential buyers for harder party drugs is more easy to identify, hence you can stop asking everyone.
It's abundance leads to a very competitive market (in Ballungsgebieten), like in other markets substitution though home production is always looming.
For dealers in Berliner parks, there's also always competition with more reputable informal suppliers as well as medical trickle downs or substitutes.
This leads to the incentive for dealers to steer consumption towards products with higher margin (or an increase of margin through lesser quality product).