Japan has a terrible demographic and zero economic growth and is doing relatively fine.

Sure, life in the future will be different than life in the past but that has always been the case. Perhaps it will (temporarily) be worse in some areas and perhaps it will be better in other areas. That'll be very hard to predict.

I don't think an economic collapse will happen, we'll have to adapt but a collapse is not in anyone's favour.

Less people does mean lower real-estate prices and potentially more nature. Perhaps we'll have to settle for less gadgets and exotic holidays but we'll have nicer homes and more nature around us.

That seems like a decent trade-off.