Many (most?) nonbelievers are convinced that supernatural believers would be better off without their religion.
That’s hard to evaluate, particularly since important beliefs such as religious convictions are not merely instrumental in letting us achieve our purposes. Instead, they strongly shape what our deepest interests are. Unless we have a way of figuring out who is “better off” independently of our particular interests, it is hard to see how we can say that the religious would invariably better off without belief. I don’t think this is doable. Sometimes we just care about different things, and that is that. If, for example, we were to find out that secular people enjoy some advantage in worldly achievements, well, the religious can rightly say they care about spiritual attainments. If, as at least some social science suggests, believers are happier, secular people could turn their nose up at what they see as cheap therapy.
But there might be some way of making headway by a more limited, more relative comparison. After all, we think of false belief as a handicap. A cognitive mistake should have pragmatic consequences. If we find a group of believers in competition with nonbelievers, trying to achieve outcomes they both care about, we might be able to find out who enjoys an advantage and who suffers from a handicap. In fact, we might be able to do even better. If we were to run into situations where nonbelievers were able to exploit the cognitive handicap of supernatural conviction and directly take advantage of believers, that would be pretty significant.
I don’t, however, see any of this happening. Religion can leave believers vulnerable to exploitation. Where Turkish Muslims are concerned, for example, it’s very common to run into financial scandals. A bunch of people present themselves as good Muslims of impeccable character, and gain the trust and the savings of devout believers, for some investment or charitable activity. They then abscond with the money. And there is the depressingly common phenomenon of sect leaders living in luxury, partly by giving the economic activity of their followers coherence, but also partly by outright exploitation.
More important, the more blatant examples of religious communities being subjected to exploitation do not involve nonbelievers taking advantage of religion. Religion is typically a hard to fake signal of commitment. It is probably easier to exploit a community if you genuinely share its supernatural beliefs in most respects. Why pay the extra cost of concealing nonbelief, if you can exploit the community and (mostly) believe at the same time? So these are not, I think, examples that can reveal any genuine competitive advantage for nonbelief.
By and large, I’m not convinced nonbelief offers any unambiguous advantage. If it were, I would expect more cases where nonbelievers successfully take advantage of religion.