It might be better for them to be religious and altruistic than irreligious and antisocial, but might there not be an alternative that's better than either?

Yes, that third alternative may be better (altruistic and irreligious), but it comes at a high cost (the individual usually loses that community, even if only from their own withdrawal) which will need a safety net prepared. And well prepared at that. I can vouch that a number of my religious friends, should they lose their religiosity, would likely fall closer to ethical nihilism, losing the altruism and being more bitter, which is all kinds of fun. So yeah, maybe it's better, but it's also significantly more costly, and the easier alternative is far from bad if it's strong on the altruistic side.