My guess will be 20%.

The majority of Americans today do not even self custody their currency. In other countries, cash still works. We can also look at trends in the "unbanked," but this example focuses on those who already use banking in their country and jurisdiction. 

The financial system will create derivatives of Bitcoin, just like it did with gold and silver, and any other commodity/asset. To gain dollar price exposure to these derivatives, and speculate, people would have to trust custody of their Bitcoin to banks. 

Many HODLers today, are waiting for some astronomical dollar price, so they can sell or leverage their Bitcoin. However, no lending entity would lend with no collateral, which means losing custody.