I don’t think you’re envisioning physics in many of these situations. Might not be the case in Barcelona which is pretty big and growable but let me give you another example to point out why that generalization doesn’t stick. I’m from nearby Sesimbra, a 5k pop village in a very very cool-looking place.
If I had to bet, Sesimbra receives like 50k tourists a month, many of which want to stay in hotels and tents and illegal caravans. If one expands Sesimbra, the beauty of the place gets destroyed for everyone. If one does not, the fisherman (yes, it is a fishing village) are now competing with 50k other fellas for land, in a village that can’t sustain any other industry.
Even if you tax the hell of tourists and only a fifth keeps going, that is already too many. It literally is destroying both the nature and the livelihood of people that were there for centuries.
I get the idea that people want to see fancy places and we don’t have that many Sesimbras in the world, but this massification is basically forbidding people from living anywhere that happens to be pretty and has a good weather. The richer tourists stay in the hotels (which lay on top of bulldozed homes) and the locals are now 20km away because physics couldn’t care less. 50k ain’t gonna stay in that tiny valley, people do not fit.
We have huge chunks of the coastal dunes destroyed (and those take a long time to recover) because of such misuse and overuse. Everyone wants to go to the beach and everyone wants to have resorts or whatever, and yet the places and the populations are taking very hard beatings. No amount of policy making solves some of those situations given that you can’t just have quotas of people allowed to visit a city within the EU. The entire region of Algarve is a disaster.
And yes, there are airports with tourist taxes, but Portugal doesn’t have all that many airports. These villages (Sesimbra, Sintra, Ericeira…) are serviced by the same airport as Lisbon. Can’t hit tourists with a tax without hitting businesses and others.