Imho it's apples vs oranges. Bitcoin can be used as currency, investment and store of value and all this by the banks, institutions and also the average Joe. This means a much wider range of entities that are bale to easily own some bitcoin. Also I don't expect banks to completely replace gold unless some more huge sources for cheap gold are found, making it useless as store of value.
All in all, it's impossible to find out a "realistic" market cap for Bitcoin. Imho sky is the limit.
Central banks don't own only gold. Many countries' central banks also own foreign currencies like Euro or US Dollar. I expect that they'll also diversify into Bitcoin. Companies have already seen its potential. Do you really think that the banks won't see it?