There is no way any institution powered by humans can be trusted to sit on a couple of billion (or trillion) dollars in liquid assets and leave them alone for long periods of time. Are they trustworthy today? Maybe. 10 years? Maybe. 30? No. 100? Very emphatic no.
Some bright spark will notice that they don't need all these untouched reserves and will do something with them, increasing the risk and, eventually, getting hit with a bank run.