Even in the ariline industry where assigned seating is common, not all carriers implement it the same way, or even at all. Southwest, a very successful operation with consistently high passenger satisfaction ratings, does not assign most seats, it boards by groups created by check in time, with the option of paying to be in the first group. Managing intermediate stop boarding and departure against early reservation check in would be more complicated than point to point seating, and while it is doablle, as many pointed out, might create as many problems as it solves.