Paid first, business, and premium economy passengers, and frequent flyer elites who get premium economy automatically would still get to preassign their seats in the premium sections,
There isn't really any premium economy on US domestic flights.
I fly WN and I fly assigned seating carriers. WN is fine for regional flights, but I wouldn't want to fly a transcon with them. I much prefer an assigned E+ seat.
The issue of getting decent seats on tight connections is also there with WN, while with a reserved seat I can get on the plane with the door closing behind me and still get my assigned aisle in E+.