Tourism is the ONLY industry for many regions... I recently lived in one. Without it, the areas would likely not exist as places to live at all.
In a country level, it is even worse as it makes a country totally dependent on tourists as an external flow of money, it usually ends up with most services been owned by big capital (local or foreign) in the expense of local businesses and it halts most forms and potentialities of actual development for other industries. Tourism can be some easy money at best, it is very often harmful for any other forms of activities and business in the long run.
Like tourists bring in "bad" consumer practices, such as buying stuff/food at "tourist traps" thus actually driving up prices, instead of them going to whenever the "locals" are used to go and still contribute to the local economy.
Best example I can think of paying overpriced shitty food at some tourist trap restaurant because you don't know better because you're a tourist, meanwhile locals go to their "known" places that aren't anything remotely similar to a "trap" you know?