I'd be careful here about constraining your thoughts to "Either magical tiny green buck-toothed AK47-wielding goblins yelling 'Wazooomba' exist, or they don't, right? So it's about 50-50 that they do." I'm not quite sure if there even is any schelling point in-between.

Complexity-based priors solve that problem. Magical tiny green buck-toothed AK47-wielding goblins yelling 'Wazooomba' are complex, so, in the absence of evidence that they do exist, you're justified in concluding that they don't. ;)