Nor does it mean this trade off for a measly 10,000 flats is worth it in such a large city.
Also, increasing density might be easy if you demolish 100 single family homes to build 10 five-stories buildings, but replacing Barcelona's 5-6 stories blocks with 10-stories ones isn't going to be economically viable. And if some brave developer tries this, then the resulting apartments won't be cheap.