It doesn't sound like a lot to me, either. I have known many people who moved to another country for graduate study. Some of them ended up settling in that country, but others pursued further study or employment in yet other countries. And perhaps the largest group among my acquaintances are those who eventually moved back to their home countries. They feel more comfortable there, they have family there, or, in many cases, returning home is what they intended to do all along.

It really depends on which 25% it is. Is it evenly distributed or is it the best and brightest, or the worst who are leaving. In addition, its institutional knowledge you are losing. I care much more about losing the guy with 15 years of experience than a fresh post-doc.