The problem you run into is how do you get $50B worth of XRP back to the starting point to be able to move the second half of the payment? You would have to wait until someone wanted to shift that amount of money in that direction.
You can’t just “give them back” because they have value. The market maker on the other end is doing something with those XRP.
The trick would be to split the buys over time. Which ruins the speed advantage of XRP. It’s possible that to move a huuuuge amount of money, it is still better to move via swift, at least until XRP is valued high enough.