Individual investors would probably not buy these. It's a lot harder to keep $100m under the proverbial mattress: not FDIC-insurable, literal cash requires guards, etc. And anything else you buy to store the value (gold for instance) has higher volatility and risk than these negative-interest bonds. So the theory would go, anyway.
Individual investors still can buy. For example, if I am sitting on too many dollars and expect that Euro will be significantly stronger than dollar in 30 years, I can diversify a little bit. Same applies to other currencies.
