Actually, I don't think this is really "the other side", just a different angle. I agree that it's everyone's responsibility to do their own planning to try to get what they need, and when things don't work out, it's not everyone else's responsibility to try to fix the situation. I believe in those cases that it is the GA's and/or FA's responsibility to figure out what (if anything) should be done.

The GA's and FA's are in a much better position than an individual pax to know how the situation came about, they have more options available to them to address an issue, they have more experience dealing with these things, and they can handle them without it becoming a personal confrontation between two passengers. The GA's and FA's can tell pax that they're just out of luck if the problem is due to the pax's own poor planning. On the flip side, if it's due to a mistake on the airline's part, they can figure out the best way to deal with it. They can also take into account whether they're dealing with an actual need (e.g. an accessible seat) or just a want (e.g. being seated together).

And if the airline does make a mistake and, for example, fails to provide an accessible seat for someone who needs it, I would expect them to reseat someone else in order to provide the accessible seat. It sucks to be that person who loses their chosen seat, but a) as we all know, seat assignments aren't guaranteed; and b) if the airline has to choose between keeping someone off the flight and putting someone in a less desirable seat, I'm pretty sure I know which one they'll choose. Obviously, they should try to minimize the inconvenience for the reseated pax, and as I mentioned above, they're in a much better position to do that than an individual pax, as they can offer an upgrade or at least try to provide a reasonably comparable seat.