>>39965
Basically
>>39972.
Youkai in Touhou require two things, one is a flat out fundamental requirement, which is Belief.
Or faith, or whatever else it could be translated as, but I think Belief/Faith is the right word that gets the point across. All non-humans actually need it in Gensokyo, some less than others, in differing forms, though that gets muddied a bit with magicians and hermits.
This is why there is a Human village in Gensokyo. The human village is a belief battery (and explicitly NOT food, despite Akyuu's thoughts. This is why Outsiders are fair game, up until they reach the Village, and why Outsiders with higher education don't typically appear in Gensokyo.) for the supernatural beings in Gensokyo. Without a mass of humans to perceive them as real, they would fade from Gensokyo like they would in the Outside World. The barrier is maintained by the disbelief of the Outside world of supernatural phenomena, while the residents inside exists through the belief of humans residing.
Youkai have a stipulation and a additional dietary requirement. The belief they need is fear (why the umbrella chick goes around scaring people. She needs* that to exist.) The other bit, because Humanity conceptualized them as monsters, is that they need humans to munch on. This is literal cooking/eating, as in tearing into dudes as you would a steak.
This bit has been muddled a bit in-universe, and you could make the argument that the Youkai diet could easily change especially in the modern era, with monsters being more and more humanized. Honestly, I have a noosphere argument to be made here, that lore wise Gensokyo's residents should be noticing friendly changes as humanity's collective consciousness perceives them in friendlier and more personable terms.
and honestly, if you look at the relationships of characters over the series, it is happening.
*And in Kogasa's case, she doesn't eat humans but the energy of humans she gains via surprises.
To note, not all Outsiders are cases of criminals, NEETs, or suicidal grandpas, and not all of those cases land in the youkai grocery stores where shit goes down (some lucky bastards might land in the Hakurei shrine. Basically EZ mode when it comes to surviving in Gensokyo.)
This is because Yukari doesn't do this by hand. It's weakened barrier shit. Autonomous feeding mechanism, basically.