Only ignorant fools will blame the downfall of Sweden on immigration. Sweden used to be a socialistic democratic country for 4-5 decades up until the 90's when the population decided that they instead want capitalism. That's the root of the downfall if anything.
Don't forget that the top 5% in Sweden are much richer now compared to the late 80's. This happens almost everywhere in the western world.
Anyway, this probably doesn't have that much to do with the state of swedish tennis. At least sweden has 4 juniors atm who's got the potential to become top 50 in the next 10 years. And should they do well in the grand slams, then surely the swedish public service (the swedish BBC if you will) is gonna start to broadcast French Open, Wimbledon, Stockholm Open, Swedish Open etc like they just to do in the 80-90's. And that will spawn more interest etc etc