i don't think they can take that much of the market share if they open up a new "fiat" based bitcoin exchange targeting US only while there is a much bigger and better exchange called Coinbase exists which has most of the market in its control.
here's my guess. they are aiming for the bittrex model, not the coinbase model: create a modest fiat gateway service that reinforces liquidity on the exchange, then focus on hundreds of altcoin markets. unlike coinbase, that's their bread and butter.
it may be an unpopular opinion, but i believe binance's USA site will still have a
huge number of listed altcoin/token markets. perhaps several dozen less than binance.com but i don't think it will be the barren wasteland everyone expects it to be.
compare bittrex to bittrex international. that's the comparison to make, not binance.je to binance.com.