How to "cash out" of the Lightning Network without closing channels?
This will get easier in the future as exchanges will be implementing deposits and withdrawals via LN.

At this time, if you need to deposit BTC and the exchange does not support LN deposits, I'm afraid you are stuck with closing channels for now.

Perhaps there will be a way soon to rebalance the channels to minimize the number of closures required.  Basically, push back funds on channels from customers 1-4, to pull the funds onto the customer 5 channel, and close only the one customer 5 channel.