As any effort banks make to shift their zero risk assets to cash won't go unnoticed by the ECB, there's also the implied risk that any bank going to the effort of acquiring unusually large piles of cash to avoid negative interest on reserves, will be subject to other direct or indirect financial penalties.
I'd still expect banks' cash holdings to rise substantially at the margin, where that risk is pretty low, but its existence means it probably wouldn't economically rational for any bank to try to evade the interest on reserves even if their cost of holding cash was zero.
I'd still expect banks' cash holdings to rise substantially at the margin, where that risk is pretty low, but its existence means it probably wouldn't economically rational for any bank to try to evade the interest on reserves even if their cost of holding cash was zero.