I'm generally a YIMBY and in favor of housing, but are we really so sure that new housing will solve the problems in cities like Barcelona or Lisbon (where tourists and people coming in attracted by the good quality of life generally have higher disposable incomes than most locals).

If you just add housing, isn't it possible that the effect is like adding roads to prevent traffic congestion? The number of people coming in just increases even more until the marginal attractiveness is the same. From the point of view of locals this means that there are even more outsiders, and the affordability is just as bad.