"the argument that if you legalise the 'soft' drugs, then there is no need for the 'hard' ones is flawed"
That was never "the argument", though.
There will always be people taking hard drugs. Face it, they are fun. And people like fun. As simple as that.
What Holland has done is to break the progression from soft to hard drugs - one is legal, easily accessible, and safe while the other is illegal, hard to get, and of dubious safety. You don't progress from Marlboro lights to marijuana, because the legal status & sellers are completely different. Similarly, you don't readily progress from marijuana to cocaine in Holland.
Yes, people also use hard drugs in Amsterdam, just like in Paris, New York, Moscow, and especially in London. Ime, London is where Class A drugs are most prevalent, much more than Amsterdam.
How much of a "problem" this is depends on your perspective. Personally, I believe in a "live and let live" approach - What people eat, smoke, or otherwise ingest is not the state's business long as they don't harm others. Again, ime, users of most drugs are much more pleasant and well behaved than drunks.
What I am trying to say is, yes, there are few dangerous drugs out there that turn their users into dangerous addicts. And their use should rightly be illegal. However, there are many others who are not addictive and are much less dangerous than alcohol, and those can easily be legalised & regulated.