|Pyro as Bill said:
London does 10 Trillion (with a T) a day in derivatives. 50-70 Trillion a week.
You're right to point out that NY is a bigger 'threat' to London than the EU but it's not like London is going to start shrinking. They might grow at a slower rate but the City is going nowhere and anyone who thinks it's success is due to the EU is delusional.
While there is certainly a risk of a lot of financial services to be lost to EU there are also a few things to consider:
- Based on brexit predictions from 2016-2017-2018 the City should now be devastated with dozens of thousands of jobs lost to EU. 4-5 years after we know that didnt happen so take any prediction with a big salt mine.
- If EU refuses to have a deal about financial services with London and lock UK out of EU financial services, there is a big chance London will go South-Asia style and become a soft financial paradise/tax haven regarding financial services (I believe this is UK objective anyway) which could compensate for a lot of the losses, if of course the professional predictions about the losses actually happen.
- If EU makes a deal the impact will be minimal to UK and still could be compensated somehow.