Perhaps [Johnson] felt that the EU simply couldn't manage without the City's financial and legal expertise and would come begging for those services to continue soon enough. If so, he is likely to be disappointed.

He is certain to be disappointed.  The City doesn't have a super secret sauce making their international finance products taste better than Frankfurt, Paris, New York, Tokyo, etc.  The work is mostly done on computers and the computers can be anywhere.  The UK is simply not as necessary in 2020 compared to 100 years ago, international firm no long have to have a presence, The City just isn't that important in the general scheme of things, and as the UK economy slowly degrades will become even less important over time.

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre