How can Tether have a daily trading volume of almost 100 bln on a supply of 'just' 24 bln? Seems rather obvious that the vast majority of crypto trades are done by HFT algos and not people (or worse, that the actual supply of Tether is much higher than the reported supply).

You can't deny that Tether is a very essential component in the crypto markets. It is also an unregulated, unaudited private entity operating outside western jurisdictions, with a long history of controversies: https://www.kalzumeus.com/2019/10/28/tether-and-bitfinex/

Hardly the group you'd want managing a global currency system.