Put another way, for the banking system as a whole, the share of deposits in total funding can increase only to the extent that investors reduce their holdings of securities and place the proceeds on deposit with banks.
There are limits to the extent this can happen since there are a range of structural, economic and cultural factors that shape the composition of a financial system, and these do not change quickly. In fact, the trend in most economies is for savings over time to move away from simple instruments such as bank deposits towards debt securities and equities.