Well obviously "infinite" is not true since there are limited people on earth with mobility to visit Lisbon, and not everyone would want to visit, and not everyone can do it at once...
But in good-faith arguments, there is a lot of difference between "build on an empty lot" and "turn Lisbon into Manhattan". As others have pointed out, there are a lot of crumbing buildings in Lisbon. You don't need "more" forever, you need "more" until demand is satisfied or supply of buildable area is exhausted (which it is not).
