*sigh*
Firstly, as has been discussed upthread, the population of the City is diverse. Whilst Brits are well-represented (obviously), this is nothing to do with any sort of British exceptionalism. It is not the Brits that are more intelligent, it is that London has attracted la crème de la crème of financial services personnel and it is fantasy to suggest that significant numbers would be willing to move en masse to any single other city or that a sudden mass hiring of new recruits would (a) be possible or (b) in any way replicate the critical mass here already.
The European Medicines Agency is finding out now that, even though people may have travelled to London from another country, that does not mean that they are necessarily willing to travel elsewhere once settled and experienced. This is partly because London is actually quite a nice city (certainly much more exciting than Frankfurt...), partly because of the widely-understood nature of English, and partly because people who came here as young job seekers are now (when they're experienced and valuable) well-settled with homes, mortgages, kids in school, marriages, friends, etc. These may sound frilly but, trust me, I have been involved in some scoping exercises for relocations and managers have consistently found that it is dangerous to ignore these factors.
No other city has the infrastructure or ecosystem to replicate London in the short term. You couldn't move everything to Frankfurt or Dublin, and if you start scattering banks in various cities, the attractiveness of those cities is further reduced in comparison. People like to work in London partly because of the choice of high quality institutions to work in. If Barclays went to Dublin, and Goldman Sachs went to Frankfurt, and Credit Suisse went to Brussels, only those people happy to commit their careers to those particular banks would even consider moving.
Secondly, you need to think about timing. The UK needs a deal quickly. It therefore is mostly interested in its leverage now. You simply cannot 'entice into employment' anywhere near enough people to replicate anything like the City of London elsewhere in the near term. Even a short absence of London from Europe would cause huge damage to European financial transactions and, therefore, the European economy. It doesn't really matter what might happen several years or decades down the line, because by then we will already have exercised our leverage to get a deal, or have reoriented our economy away from Europe.