I think the South West model is a total disaster. It has the worst features of Amtraks open-seating, and reserved seating used in other transport. First of all, it is not really open-seating. It starts with the business-select group, then the upgraded boarding group, then the early check-in group. These tiers all cost more money. Then "families". Then in order that everyone did the normal check in.
Not every tier costs extra money. Here is the order and cost of each group to the best of my memory.
Disabled People ($0)
A01-A15 = Business Select (Full Fare + Drink), Anytime (Full Fare), or Upgraded Boarding ($30-$40)
A16-A30 = A-List and A-List Preferred (status based)
A31-A60 = Early Bird ($15)
Families with Young Children ($0)
B01-B60 = Timely Check-In ($0)
C01-C60 = Late Check-In ($0)
If the flight is full, there's a cancellation or change in scheduling, or you're buying at the tail end of the APEX/WGA booking period, then everything beyond A15 can get pushed further back but I've never had Early Bird drop me further back than Group B. That usually allows for an aisle seat toward the back while Group C is almost always stuck with center seats.
So many people try to game the system that its far, far worse then amtrak. I've seen groups of 9-10 with ONE child, hoping to jump to the front of the line. I've seen groups with ONE person pay for priority boarding that hold seats until the rest show up. Or they do the "puffy jacket", or "fake sleeping" tricks you see on trains. Or the person who jumps the line, and pretends to be confused, or convenient lack of english.
I've flown many Southwest Airlines flights over the years, in fact I've flown almost every nonstop route they offer from my home airport, including some they no longer offer, and yet I've never had much of a problem with anyone trying to cheat me out of a seat. I'm normally in the A-Group but even if I was in the B-Group I'd simply ignore whichever Kettle Family was trying to game the system and take whatever seat was open. If they want to make a scene about their lack of planning then so be it. If I was in the C-Group I'd simply accept that my seat is going to be crap no matter what.
Southwest Airlines doesn't make the extensive intermediate stops which Amtrak does
On many routes Southwest does indeed make several intermediate stops with the same aircraft hopping from city to city in a chain from one end to the other like in the old days. That's how you can end up with several rows of able-bodied people already seated prior to boarding A01-A30. The benefit to passengers is that Southwest offers lots of nonstop flights without having to connect through some massive fortress hub and their walkup fares are a small fraction of what the legacies previously charged.
While I am very far from an expert in the area of airport operations, I would also question whether the Southwest model is actually the fastest way of loading a plane and even if it were, whether it really makes that much of a difference overall (too many other sources for delay or which just take time on the ground). If turning the plane quickly is so dependent on passenger boarding, seems like you'd have at least long-term plans for terminals to load from more than one door.
Southwest spent several years testing different boarding systems at various airports and the changes they deployed nationwide was the fastest and most efficient option available that didn't require extensive remodeling or retraining of staff and customers.