All cash is, also any investment below inflation - so most bonds.

This motivates productive use of that capital. Deflationary currencies just guarantee the rich get richer as an axiom.

> This motivates productive use of that capital.

Nah. It motivates rational beings to spend that capital on anything at all, as fast as possible, because the alternative is a net loss of purchasing power.

Deflationary currencies on the contrary motivate actors to think before spending, because any spending has to be balanced with the future gains that won't come from holding it and seing it accumulate value over time.

The real solution exists somewhere in between an inflationary and deflationary currency. This is a balancing act.