There is plenty of successful people ready to step in their place, are university's are packed with talented ready to embark on good careers.
And who do you think work in top UK universities, doing world-leading research and training up the next generation of talent?
In my field, it's about 50% international staff from across the EU and to a lesser extent North America and Oz/NZ.
If even a sizeable minority of these folk drift away over the next few years because the UK outside the EU is a less attractive place to live and work, the quality of universities in the UK will really suffer as there simply aren't enough UK academics of comparable standard to fill the gap. (Of course there are great UK academics; just not enough of them to staff the all top universities).
And when the universities in a country start dropping in quality, no longer producing world-leading research in such volume, closing courses because the departments that teach them aren't raking in enough research income any more... then the long-term economic impact on that country is negative and huge.
My university is bricking it in the even of a Leave vote. The vice-chancellors of most of the top universities have collectively called for Remain, for very good reason.