The mere transfer of money for a fee cannot justify the extraordinary gains made by cryptocurrencies.

Agreed. But you asked about the revenue stream for liquidity providers. They're not getting rich on their USDT.

The non-stable coin prices are speculative, bearing no relation to the cost of producing a coin or the money one can make by owning coins. The money transfer business may be profitable, and to engage in it, a broker needs "inventory.” A certain discounted present value of future cash flows can be associated with each BTC held in that inventory. That is its value. At this point, however, no one seems to care about that measure.

As a currency, BTC is silly. But as the verifiable, universal translator of any currency sent to any currency received, it is a useful communication/transportation asset. The business is money-moving, and BTC are tools someone in that business must own. Bitcoin only looks like a currency. Actually, it's a wrapper for currency.