I get that, and it's an age old accusation against speculation. But it's really not so simple, because limiting the supply also has costs and it might ultimately fail (it usually does).

Do you have any numbers that show the speculation is really what is driving the prices? I doubt that some vacant flat here or there has a significant impact on prices.

If a street was completely vacant, prices would probably go down because it would be unattractive to live there.