In my experience (please don't take this as me saying this is the way it always is...) families still are seated together MOST OF THE TIME. In the case that they aren't, I have always experienced Delta to be very helpful in asking for someone to switch. Nobody is forced to buy basic economy and they make it very clear what you are getting yourself in to. Clearly there is a market for it and families continue to purchase those fares so something must be working right.