Is this a plausible scenario? Suppose I open a new account on Bittrex. I transfer some BTC to Bittrex from an exchange where I bought the BTC with USD. Once the BTC arrives on Bittrex, I exchange it for Tether to buy another cryptocurrency. Bittrex then takes my BTC, or a portion of it, transfers it to an exchange that allows a USD exchange and then transfers the USD to a bank account. Allowing for transfer fees and slippage, the final USD maybe slightly higher or lower than a 1:1 exchange rate depending on the price of BTC at the time of final conversion to USD. 

Under this scenario, why couldn't Tether be a solvent intermediary and Bittrex be consistently profitable whether BTC price goes up or down?

Is this too simplistic? Am I missing something here?