I'd think in terms of how much it costs to hire these people and what Fidelity's profit-per-employee rate is, as that should in theory give an idea of a lower b

I'd think in terms of how much it costs to hire these people and what Fidelity's profit-per-employee rate is, as that should in theory give an idea of a lower bound for how much they expect to earn from crypto. One would expect trading volume to be several to hundreds of thousands times that amount, assuming fees similar to Coinbase/Kraken/et al.