Investment in mining hardware whose value is tied to a speculative asset without fundamental value beyond utility.

None of the current implementations can scale, so even their utility is questionable. There is a way to make a scalable trust-less digital currency by using a mutable leder, but none of the current coins that I know of are heading that way. The main difficulty will be how to distribute ledger space in a way that cannot be monopolized by the richest. That is a simple technical problem to solve though, because storage capacity will eventually surpass the ratio of richest to average wealth. Then you can allocate ledger space in exchange for an affordable deposit, redeemable when the balance is emptied.