Ok, so how are you making money to sustain your operation? Presumably you are a business, not a charity. Your vault isn't free, neither is the land it sits in, and as you gain more customers your security costs are going to increase. Also your customers are going to want convenient withdrawal and deposit options, so you will need multiple vaults in different locations (plus security measures). Not to mention the overhead you'll need to run this business (accountants, HR, IT, asset managers, etc). Thus there is a degree of scaling to your costs. This is fine though, just subtract these costs from the revenue generated by your investments. Your simplified business model looks something like this:

Profit = Investment Returns - Operational Costs - Interest Paid to customers

Only you have to have sufficiently liquidity such that your customers can get withdrawals whenever they want. This means your investment pool is going to be conservative, thus limiting your returns. Now what happens when your nation's central bank craters interest rates, thus driving your investment returns down? Oh shit, now the revenue source is drying up. What gives? Profit, or your interest rates? Well profits have to be at minimum high enough to justify all this effort, so looks like the interest rates are going to drop and drop and drop as the cost of money goes down....

Congratulations, you are now in the same place as all those other institutions. Who would have guessed that an identical business model subjected to all the same economic push and pulls would end up at the same destination.