To be honest i have no sympathy for immigra,,, er sorry expats. It is pure location arbitrage: rich pensioners moving to south europe to get a vastly bigger bang for their buck than they would get back home. It would eventually backfire, and i am glad it does. It was idiotic of the touristic countries to invite this kind of thing in the first place.
I am not sure what is wrong about that: they spend their money and that money falls in the hands of locals. I do not mean it should be the priority of a country (it should be other economic sources) but I do not find it harmful.
when there is location arbitrage, then someone is benefiting at the expense of someone else. Practically, expats add to the pensioner population of the country (except for the burden of paying their pensions). European countries are designed to require large working populations to sustain public services (which older people will disproportionaly use more). Add to that the unintended consequences such as high real estate prices. Therefore i believe that overall, it harmful.
