If you have a city with X housing units, but 5X people from within the city + all over the country/world want to live there, and for whatever reason no one wants to/can build 4X more housing units, how do you fairly determine who gets to live there and who doesn't?
* Is it fair if you were born in Lisbon you get to keep living in Lisbon?
* Is it fair if people early in their career get to live in Lisbon because it will advance their career much more than if they lived in the countryside?
* Is it fair if a rich person gets to live there because they can pay a lot of money to do so?
Whatever policy is in place, it will be deciding the winners and the losers, or the people who get to live in Lisbon and those who don't.