Currency "investors" are always speculators; they are not buying a productive asset of any kind.
This is misleading, in the sense that currency is a very productive asset. It doesn't produce more currency (unless it is a staking vehicle) perhaps, but that is almost a tautology, content-free. It enables the whole economy of that currency. Calling that non-productive would imply that all economic activity was non-productive.
Were there no efficiency and growth-enabling features in a new technology (currency or otherwise) it would fail to thrive in a competitive landscape. An investment in a novel currency is an investment in the whole economy of that currency. It adds capital to the economy, which helps it to achieve a productive critical mass, and the growth of that overall network is what the investor is betting on. Because it is an investment in the totality of the economy, it is maximally diversified, relative to that economy - sort of like indexing on steroids. The whole world becomes your productive asset.
