Yes, that lower bound is a function of all those things: security costs, labor costs, etc. They can be laboriously approximately quantified. But it surely exist

Yes, that lower bound is a function of all those things: security costs, labor costs, etc. They can be laboriously approximately quantified. But it surely exists: for example, it's inconceivable the rate to ever reach -30% because storing money will never get that expansive in normal conditions (please discount apocalyptic scenarios).