Proof of Liquidity > Proof of Reserves.
This is the way for the new banking system, where there is a global denomination (sats) and every bank is subject to a potential bank run at all points in time. The only way to survive in this ecosystem is to be perfectly liquid and perfectly honest. A bank can be fractional if they wish, but if it impacts liquidity, they are immediately branded as such, and the funds flee until the rug is pulled. The system naturally self-calibrates.
This is how the banking system used to work in Scotland and Canada, until central bank monopolies took over.