I think most college towns are pretty similar in that if you have an academically focused family (like I did, and many do as children of UO faculty) then it's pretty much standard that the child will look somewhere else for undergrad because UO isn't very prestigious in that regard. And those places are usually all over, quite far away. I went to NYC for school and I've stayed here since, and don't know anyone that's moved back to the Eug after graduation.
I think it's also because Eugene has such a progressive, hippy/outdoorsman culture. We grew up with people like Ken Kesey and Phil Knight and loggers and like even Christopher McCandless as our local heros. So there's a sense of eccentrism and adventure that's important to the culture there, so I know a some people who left to travel the world or become full time rock climbers.
I haven't lived there for over five years so I can't really say how that culture is changing. I think real-estate prices have been rising steadily for a while as it becomes a 'cooler' town; people moving from Portland for a smaller but similar taste of Oregon weirdness. It definitely is somewhat undiverse, only small Latino and other non-white communities, and that probably is not changing considering the Portland-Eugene migration. But overall it's a good place for instilling a hippy-progressive-academic sensibility in kids, and I have a lot of nostalgia for it.
Also congrats! I applied to Mississippi for fiction. So in some distant, unlikely universe we'll be cohort buds