The wording on SW's preboard policy is that it is for anyone who needs extra time boarding, assistance, or needs to sit in a specific seat on the plane. I know this, as I am one of the ones who needs a specific seat, although you'd not know it to look at me, or watch me when deplaning. I need an aisle seat on the port side of the plane. My right knee only bends about 60degrees (poor outcome for knee replacement surgery), so I really, really try to get that aisle seat so I don't have to force my knee into a painful, bent position for much of the flight. There's not a lot I can do during beverage service, and I try to shift when people are walking in the aisle. The best solution for me is to get bulkhead seating, but this is not always possible.
I don't have a good solution for SW. I kinda like keeping the open seating policy, but change pre-boarding to assisted boarding and do it right after the A group, before family boarding? I also like the idea of keeping family boarding in the back of the plane, just because it would be the easiest way to hold entire rows of seats open, as well as overhead bins, so families could be guaranteed of a better chance of sitting together. I don't know. Don't think there is a good way of doing any of this. It's either keep the current system, or get everyone to pay for all front row/aisle/window seats, I think.