I assume you just find rock solid corporate bonds. Apple issues bonds and they have assets far in excess of their debt load + massive profitability. Realistically as an individual you just move into equities. Institutions have different equations. Hoarding physical cash is probably not a good idea, since you have inflation (which probably exceeds the negative rate by quite a bit). It’s worth mentioning that cash holdings in a bank already have negative real growth because inflation exceeds interest (at least in my shitty low interest savings account)

Hoarding highly liquid physical assets like gold or paper requires security which costs money. The negative interest rates are cheaper than buying equivalent security.