I agree with you where you say that currencies are worth only what peoples' expectations of its future valuation will be, but that is basically a mechanism for

I agree with you where you say that currencies are worth only what peoples' expectations of its future valuation will be, but that is basically a mechanism for valuing anything - not a description of a pyramid scheme. A pyramid scheme is one where there is a continued and ongoing transfer of wealth from new scheme joiners to earlier scheme joiners, which works great until the supply of new joiners dries up - then the thing collapses. A new currency could work (in theory) absolutely fine with no further issuance, and no new joiners beyond the first round of adopters - not at all the same as a pyramid scheme.