In many cases, I suspect that people adopt false beliefs and the ensuing dark-side for short term emotional gain, but in the long term the instrumental loss outweighs this.

Not only this.

If people never adopt the map that corresponds the most to its territory, they'll never have an accurate cost-benefit analysis of adopting false beliefs.

Maybe, in some cases, false beliefs make you better off. The problem is you'll never know that, unless you first adopt reason.