Moving from Venice to Florence as the city to discuss, what would keep the city center for the locals if tourism wasn’t the main driver passing people out?

I'm not moving the city to discuss, I spoke of both cities as an example of overtourism. I simply used Florence in later examples because I know the city better than Venice.

We’re seeing a lot of cities seeing an influx of “expats” seeking lower cost of living areas to work from. Florence has a beautiful city center with a nearby high speed rail train station and decent airport connections. There wouldn’t be anything stopping the locals from getting pushed out by digital nomads and trust fund kids who want a year abroad.

You still don't understand how different is to have someone living in a city for a year and someone living in a city for a few days? The scenario you are painting is way better than the overtourism because someone living in a city for a year still needs a supermarket. A tourist doesn't. Just to name an example.

Yes, over tourism is what is currently changing the city, but I’m not sure that the same thing wouldn’t happen anyway, but with the rich guests staying for a few years instead of a few weeks.

Someone living for a few years in a city needs services that a tourist visiting for a couple days doesn't. And, most important, keeps rent available and at a affordable prices. Someone living for a whole year will occupy a house for the whole year, so they'll get a normal rent. That means the rest of renters will have to keep prices the same. Also, someone living for a year there, will need to declare residence if they want to use services like, for example, doctors. That also means they'll have to pay taxes which, in turn, are used to improve the city. A tourist does nothing of that. And since 2-days rentals are so popular, nobody will rent their house to someone living a year when in a couple of months of BnB they'll make the same money; driving most of them out of the long-term rental and making the few remaining raise prices absurdly.

On top of that, there's the question of the general degradation for the city that overtourism brings. Pickpocketers, scammers... they are all lured by tourists, so the more tourists, the more of these. I know, I was here before the boom, and things weren't nearly as bad. Overtourism also bring a category of tourist that's rarely careful or considerate with both the place and the locals because for them it's all a theme park and, since they come for just a couple of days, they don't even care about littering or about local customs...