It isn't an original idea, that all action is ultimately 'selfish'. Writers such as Ayn Rand and Richard Dawson put forward that idea decades ago. However, the summary of the philosophy has important parts missing:
"All human action can be seen as ultimately derived from the maximisation of satisfaction and the minimisation of discomfort, as perceived by the individual at the time".
Note that this does not rule out 'altruism', because 'altruistic' behaviour can be seen in terms of non-material rewards an avoidance of discomfort. A common reply to a question of why a person did something 'altruistic' for instance is "It makes me feel good". Another is "I would feel guilty if I didn't do it." Religious people often express it as an expected reward or punishment in the afterlife.
The critical thing to evaluating people's behaviour using an 'economic' model, however, is finding out what 'weight' the individuals place on each action, and that is where such models break down because rarely does even the individual concerned know exactly what they are evaluating. The 'weights' can also change from moment to moment, and often do not reflect reality. People can and often do go for an immediate pleasure over a long-term good, where at another time they may think more rationally about the long-term effects and forego the immediate satisfaction. And of course something I find satisfying or painful may have the opposite reaction for someone else.