Nobody was suggesting you would be removed from a flight if somebody else was occupying your seat for whatever reason. The removal was a reply to a hypothetical question as to what happens "if I refuse" etc. That is why words such as " how long is a piece of string" and "common sense" were used.
The same criteria would be applied if someone refused to take their seat because they were sat next to a baby or a woman. A common sense solution would always be sought, but the question was posed as to where the ultimate judgment would rest and that is why the answer was given.
In all likelyhood, a passenger would have to be able to fit in to their designated seat, or be required to purchase a larger seat or another seat. However there may well be jurisdictions and specific situations where that is not the determined course of action. Similarly the decision as to action taken over the degree of inconvenience or nuisance, will not rest ultimately with the passenger.