Housing should be a national interest and something that we work to make as obtainable by the largest portion of our citizens.
Owning multiple houses and flipping to keep the market surging should not be legal.
They've been priced out of the market, and that's okay. This process is exactly what grows populations in other cities.
Without private property ownership, the entire rental market would cease to exist. Often people purchase homes in disrepair, fix them up, and rent them out. The rental income is precisely what incentivizes this process. If there was no yield, people wouldn't do this and there would be even less housing available.
Anecdotally, I believe in Amsterdam there are restrictions such as a house can't remain vacant for more than x months. There may be some merit to that, but I still really dislike it. It basically says "you don't own your private property".