If 10 users all buy $2000 in Bitcoin, they earn $200 fees in spread.
Now when we withdraw they pay a $100 fee.
Still $100 gross profit, down from previous $199
Discussion
They still group withdrawals into big ass transactions with multiple outputs, so even like that, it won't hurt them as much. With the model you describe they'll feel it more than other exchanges, for sure, but the spread of a single big buy probably pays many times the fee.