I'm curious if all those decriminalizations and legalizations in various states & countries are having an impact on Netherlands' tourism.
But yes, the actual number of cannabis connoisseur tourists is tiny.
Hardly unique and recently greatly outshone by the offerings in states in the US where it's legal.
If someone is willing to spend thousands of dollars to travel to the Netherlands because of marijuana, they are probably pretty into marijuana, and if they are that into it, they are probably using it in their home country anyway, despite it being illegal.
The legal barriers are not hard to work around, and in almost every country it would be easier to get illegal marijuana locally than to arrange travel to the Netherlands.
It's mostly just part of the country's (and Amsterdam's in particular) reputation. Let's put it this way: you can legally obtain cheese and wine in most parts of the world -- even French cheese and wine -- but that doesn't stop people from associating France with cheese and wine (whether when in France they actually consume it or not).
It was so bad that the mayor of Amsterdam noped out of introducing it, together with most mayors in towns that doesn't kiss a border. I believe most, if not all coffee shops in the Randstad area (Amsterdam, The Hague, Rotterdam, Utrecht) are still welcoming any adults into their coffee shops.