The problem is that guaranteeing a bank and regulating it's investments still changes the incentives for the bankers. The banker has an incentive make an invest

The problem is that guaranteeing a bank and regulating it's investments still changes the incentives for the bankers. The banker has an incentive make an investment that can presented as "prudent" but which actually has a large up and down side. The banker keeps the upside, the bank's depositors are protected from the downside and the worst the investors face is losing their existing capital.