Yes, in positivist sciences 20% intending to stay would be very high by historical standards.

This paper https://dspace.mit.edu/handle/1721.1/93208 gives and estimate 87% PhD holders leave before becoming (tenured) faculty. And that's academia-wide. In the sciences more will be leaving. In exact sciences yet again more.

Truth is most people leave before even getting a PhD, so it's even worse (and the advice is to think long and hard before doing a PhD, and certainly starting one because you can't find a job for a few months is sure to result in disappointment)