not quite. banks dont want cash (see pg17 of JPM Q4 results,...

not quite. banks dont want cash (see pg17 of JPM Q4 results, where they stated theyre going to turn away deposits or charge neg rates on them), they have too much reserves already and they'd need to place it at the Fed which gives them no benefit.
SLR isnt a risk sensitive measure, like RWAs that require high levels of capital. its a notional one. so youd move low risk low return USTs to free up room, cutting risk exposures dont help. all analysis in the area says the expiry of the exemption is a non-event as it was only ever applied at the HoldCo and not the Op entities. if it was going to show up it will appear in the repo market first, like Sept19. the benefit it provided was relief last year in letting GSIBs soak up USTs, but they've all got room in their lev ratios at the moment. see Zoltan Pozsars work if you want to get into the detail.
The real impact of the expiry of the HoldCo SLR exemption is that there is limited capacity for most US GSIBs to absorb too much more USTs (as they want to buyback stock and pay divs = reduced cspital = reduced leverage capacity) at a time when they are going to be issuing at increasing pace, which means the FED is going to have to monetise more of them than they have been of late. i dont think people quite grasp just how much this will be.