This has very little to do with nomads and everything with tourism.
15 years ago Lisbon was in terrible shape, mostly collapsing due to rent controls.
And this is still the case to a degree. You have an enormous number of houses where the (older than, I think, 55) tenants pay 50 Euros per month that cannot be increased and the landlords pay more than that just in taxes and basic fees. So no renovation is done, everything is falling apart.