Isolation doesn't work. You may think you are the best in the world, but if you don't constantly measure yourself against others, learn their styles and incorporate their techniques into your own, you won't improve. England thought it was the bee's knees when it came to soccer for much of this century, which is why it didn't enter the World Cup until 1950. And when it finally did dip its toe into the big pond, it was beaten by a bunch of part-timers from the United States. What it didn't realize was that by not playing anybody outside its country, it didn't evolve one bit.
Foreigners bring new ideas, new approaches and, often, a whole new set of skills.
Their presence shows youngsters that there is more than one way of doing things and, often, they emerge as better players themselves. Before coming to Europe, Diego Maradona was simply a footballing genius. After his years in the Primera Liga and in Serie A he became a footballing genius who knew how to deal with rigid tactical marking. When clubs can draw upon 10 international-quality players, it is better than having three. It means more depth for the top teams, but also it means more talent is spread around.