Wolf-Georg Ringe has written a paper which argues that the impact of Brexit on the EU financial markets may be minimal. In a blog describing his paper, he explains how past examples demonstrate that:

legal principles are easily set aside when economic exigencies so require. This experience leads us to predict a similar approach being used for accommodating Brexit. My paper therefore predicts that a specific, tailor-made agreement will be concluded between the UK and the EU 27.

Read the blog post, which links to the paper, here