The problem with that is that it becomes a self-fulfilling prophecy. If you assume bad faith, you're more likely to act in such a way yourself, setting a low standard for the foreseeable future.

A lack of trust will also restrict your options. You'll spend more time making sure all contracts are watertight, or have to pass on opportunities if the legal system (which is the fallback option after 'trust') isn't capable to compensate.

It's also somewhat wrong on the facts: The idea of completely rational and selfish behaviour is called 'homo economicus', and it's among the worst of all the bad assumptions economics has made. Because as it turns out, it's almost impossible to find people acting in such a way outside of mental institutions.

The only rational and selfish test subjects are usually monkeys. Humans are willing to forgo all sorts of rewards for either altruistic or moral reasons.

The issue is not really so much if it makes you an asshole or not. The issue is whether it makes you effective or not. Will you (and other people) be better off if you change your behaviour? Personally, I don't know the answer to that question, but I think it is a very interesting one.

Everyone assumes that everyone looks out for themselves first. That is not a controversial or novel idea.

The question is whether, in a team situation, one assumes that others are looking out for the shared interests as a group. If no shared interests exist, then the team has a much bigger problem.